Is it normal for your thighs to become fatter while on your period?

Your diet sounds like it is pretty meager, I'm not sure you are getting enough calories. The exercise you are doing is great, but I would add a work-out with weights, that will really tone your legs up fast. Also check the soup...how much sodium is in it? And the Greek dressing...did you make it yourself, or is it from a bottle? If purchased, check the sodium content in that. When you have your period you retain water anyway, and combined with sodium you may be experiencing a lot of bloat. Oh...I just say your additional details...watermelon contains a lot of water...this combined with all the other stuff is probably what happened. I wouldn't stress.