There is a medical reason but it's not something doctors understand how to get to yet. There are all sorts of chemical things going on and who knows what one chemical is too high and one too low or something. I posted in the depression forum about how depression, anxiety, and other mental problems might be related to thyroid hormones. They just don't know enough yet to do "individuals" very well.

How old are you? Are you going through perimenopause or anything like that yet? Could be the result of a week's worth of not eating well, your cycle (I had the most trouble with my anxiety in my 20's and 40's because of those two, my cycle/not eating right and then preimenopause) all sorts of things can go wrong that we can't "see" yet and cause anxiety/panic attacks.