I get this too. It started recently, on holiday, when I was feeling far more relaxed than I have been lately. I usually get it in the evenings or just before I go to sleep. I know it must be another anxiety symptom and I'm not actually too worried about it because I have realised it only happens when I'm not doing anything else - I never get it when I'm busy!
I just don't understand why it has started now when I am much less anxious than I have been. It's like there's something in my body that wants to kick me down every time I feel I'm back on top of things - just to remind me that my anxiety is still there, if you know what I mean!