"I am such a creature of habit that I eat the same thing almost every day and am positive I haven't eaten anything new."

Perhaps you've developed an intolerance to some food you're eating a lot of. That happened to me with soy - I also was eating pretty much the same stuff every day, and it included a fair amount of soy. After about 10 years of this my body started freaking out (itchy, painful rashes) and it took a long time to figure out what was causing it. (It took an elimination diet for me to finally get to the bottom of it.)

Something for you to consider, anyway.

Good luck!