Wellies emotionally abusive people use silent treatment as a punishment. A lot of them also project everything about themselves onto others (whether they also recognise whatever-it-is in themselves or not). So because they'd use silent treatment to punish you, when you go NC they think that's what you're doing. NC isn't silent treatment though, but they don't realise that. NC might look similar to silent treatment at first and on the outside, but it isn't punishment and there's a totally different mentality behind it, it's purely a self-defence mechanism.
Cats you don't have to work out what she is. You can't actually diagnose someone else anyway, you're not a doctor. Even a doctor can't diagnose someone they're a close relative of or who they've never seen or spoken too. At the end of the day, whatever we work out - it's all just opinions.
You can also spend your life trying to understand someone and their behaviour but at the end of it all, what has that actually achieved, other than to use up your own time/energy/headspace? Understandably, we sometimes feel compelled to try to work them out anyway. Just don't worry about it if you can't.
All you need to work out is how you feel and why, that's the start of self-growth.
When society lays down it's expectations of how things should be and makes moral judgements, those expectations and morals are based on healthy interpersonal dynamics. The underlying implication is that doing these things doesn't harm you, so not doing them means you're selfish. Dysfunctional dynamics don't need to (and in many cases would be harmful to) follow the same pattern.
Think of something really basic and black + white. Spouses should support each other. That's generally accepted as being an accurate viewpoint in society. So when one spouse is a paedophile and has abused their children, should the other spouse support them? Hell no, they should support their kids first and foremost, their own health and wellbeing second. Any support they give their spouse should be at a distance, such as paying for them to have therapy, but only if doing so wouldn't result in any harm to the kids, like them growing up going without this-or-that (including their own therapy) to afford the therapy for the paedophile. In reality most people wouldn't have the desire to support the paedophile at all and would simply divorce them and go NC. Society wouldn't berate them for this, even though they're being an unsupportive spouse, because it's justified.
The issue comes when a person doesn't recognise that what they've gone through amounts to abuse or neglect, or just plain old disrespect, and so thinks they'd be going NC "for no reason" so doesn't feel justified in doing it. You have your reasons and they are definitely "enough" to justify it. When you're in a situation like yours where you can't look after the other person's wellbeing without harming your own, always choose you.
Selfishness is: not considering others and their needs or deciding that your wants come before their needs. Selfishness isn't the same as putting yourself first. The former is a personality flaw, the latter is necessary for health and wellbeing.
It's nuanced.
Imagine if you're all home from a family day out and you're hungry, who gets fed first, the dog and the baby or yourself? Ordinarily it's the pets and kids, because they're dependent on you, to feed yourself first would be selfishness.
But what if your situation isn't ordinary? What if you're diabetic and you know you need to eat right now because you're not feeling well? In that scenario feeding yourself first would not be selfish, your need is greater than theirs. To feed the others first would be foolishness and possibly put their safety at risk, because it would delay taking care of your needs which risks your health and you can't look after dependents if you're on the floor passed out in a diabetic coma. Whereas if they had to wait half hour to eat they'd just be more hungry and barking/crying, they'd be upset but not physically harmed. It isn't neglect.
But if you fed yourself first in the "ordinary" scenario when there's no reason you couldn't wait, then you'd be causing your dependents unnecessary distress making them wait, which is neglectful and selfish.
In your situation with your mum, she's causing you emotional harm. It's not an ordinary situation. Staying away isn't selfish, it's protecting yourself from harm. Nobody is emotionally harmed by a boring person. It's boring being around them, that's all and although it's joked about - nobody ever actually died of boredom. If you're coming away from visits and phone calls emotionally distressed, there's more going on with her than just being boring. Even if she was just boring, if there's been abuse and neglect in the past then you don't owe the person your time of being bored. If you're not responsible for the person then you don't owe them your time of being bored, even if they've never done anything to you. If your mum wants someone to talk to she can join a social group and make some friends.
Sorry, I've rambled on a bit.
Anyway, nothing has to be permanent. Give yourself breathing space. You're not setting down your entire life's behaviour going forward, engraved in stone and legally binding. You're just making a decision not to call her or visit her for now. Take it day by day or week by week. Focus on taking good care of yourself and reassess your decision as often as you feel is right.