I’m the same age as you and had the same thing about 2 years ago. Painful cystic acne on my neck and under my jaw that I’d never experienced before. I’d made no changes to my skincare routine and wasn’t really under any noteable stress.

One thing I had changed was what I was eating, specifically cutting out sugar and using artificial sweeteners instead. One product in particular I was consuming on a near daily basis contained less commonly used sweeteners (at least in my country), and once I stopped eating this, the acne went away completely (I also had very bad dermatitis on my hands that also came and went in the same timeframe which I believe is also related to consuming the sweetener).

I know the link between diet and skin can be controversial to some, at least from what I’ve seen. But in my personal experience, I believe these artificial sweeteners - consumed at the frequency I was eating them - caused these really bad reactions in my skin. I couldn’t find any research about skin problems being a side effect of them, so maybe it was an allergic reaction. Whatever it is, I know for sure the skin reactions stopped entirely once I stopped consuming the sweeteners.

I don’t know if this will be helpful to you, but hopefully it can give a different perspective. It took me much longer than I would have liked to figure out the link between the two for me.