Well, the first thing that comes to mind is did you do something you haven't done for a while - like mow the yard, lift or push a lot of stuff using your legs as a help - It is POSSIBLE, you just strained your muscles. Just walking a lot CAN do it if you are not used to it. I have been so sedentary since all this began last October that I am finding my muscles need retraining. If none of those type things have been done recently, then I would probably think it could be thyroid related - but I have no experience with it.
Here's hoping it is just overuse of muscles!