OP, there is always a reason why people reach a high BMI - or more likely, a whole load of reasons. You have listed several.
I suppose there are some people who simply choose to be big because they like it. But in the overwhelming majority of cases, it is not so. Some people naturally carry more body fat than others, some are naturally more muscular. And a very large proportion of us fall foul of other things - the food environment, sedentary lifestyles, medication side effects, comfort eating shading into eating disorders - which, by the way, are largely caused by dieting.
Whether someone has always had a high BMI, or, as in your case, acquired it through recent life changes, is IMO irrelevant. A high BMI, in and of itself, is not a reason to be ashamed, or for others to shame you. It is a reason to ask questions about what has led to it - without judgement, with respect and compassion.
You have mentioned PND - are you receiving treatment and support? RA - I imagine pain would make physical activity difficult, and do the treatments for your RA predispose to weight gain?
And comfort eating - been there, got the T-shirt, in fact I have Binge Eating Disorder. If that’s a significant factor in your weight gain, I gently suggest you explore ways of improving that, rather than think “I have to lose weight” and go on a diet. It tends to be counterproductive, especially if your relationship with food is already dysfunctional. I thoroughly recommend Intuitive Eating by Evelyn Tribole.
Be kind to yourself. You’ll find plenty of people who won’t be.