Go see your doc to find out what it is, or at least what it isn’t.
Your story is probably pretty typical in that riding more than an hour causes no discomfort but can’t even take a backswing to not riding and now you can take easy swings. In other words you have no clue.
And not to throw a curve, but if you take a statin, that can wreak havoc. I’m finding out cutting my normal dose in half has helped also. Should talk to a doctor first... do as I say not as I do.
John