Oh, Cnmorgan this sounds so familiar.
I'm no doctor. I do, however, have OCD, and this is very much how I get sometimes.
One of the characteristic things about OCD is that the thoughts are intrusive, and it can feel impossible to shake them. They might bubble up when you're in the middle of doing something else, or they might be constant and low-level, like a rumbling in the background. For me, I had patches. Low level generalized fear, and spikes of true terror. It was awful.
You haven't asked for advice, but I want to urge you to see someone qualified to diagnose you.
I suffered for years trying to cope, but asking for help was the best thing I ever did for myself. I went to my GP and told her what was going on (honestly, she wasn't very much help but she gave me some resources and they helped) and then got a CBT therapist. After the first treatment I felt better. Just knowing my abnormal thoughts were totally normal for somebody in my position, and I wouldn't have to keep living like that? It was wonderful.
Please do see someone. I suffered so long, and I wouldn't wish it on anyone. If you want to ask any questions, please go ahead and ping me. As I said, I'm not a doctor, but I am happy to talk.