In my personal opinion, I think you are needy, and have an unhealthy attitude and expectation of friendships (at least in regards to how quickly someone should reply to you).
I know a lot of people have time on their hands during lockdown, but plenty of people are still working full time, running round after kids (even more so with homeschooling) and quite frankly, maybe just needing some time to themselves, and want to scroll facebook but don't want to reply to private messages at that time.
I quite frequently access social media, whilst I've got unread, or unreplying to private messages. So do many of my friends. We reply when we want to, really. Sometimes I want to just occupy myself for a bit, without having to have a conversation.
I also tend not to send (or reply to) what I'd personally see as pointless messages such as 'had a headache, hope your weekend was good', 'hows lockdown treating you' - I personally find it bland and rather dull. Plenty of people do enjoy general chit chat and that's absolutely fine and quite nice. But personally, I can't be bothered with that, perhaps that makes me (and my friends who are similar) bad friends. We seem pretty happy with the status quo. We generally only really say something, when we have something to say. It can go a month or two between speaking with some friends, and then we'll have a good catch up.
Some folk prefer to keep themselves constantly busy with hobbies etc.
There's many reasons for it, and all of it is fine. If this is new behaviour in the last year, try thinking outside the box. Is she an essential worker? Has she been homeschooling kids? Could she be struggling with lockdown and withdrawing as a result? You seem to preoccupied with being offended she hasn't responded, that it doesn't seem you've considered what could be responsible for the change.
And if none of the above apply, maybe she has just changed a bit. That's fine too.
If one of my friends called me out for accessing social media/having read their message but not replied for a while, I'd also feel extremely creeped out that they'd been keeping an eye on me, and checking up on me. I'm pretty certain several of my friends would feel the same if I did that to them.