since muscle weighs 3 x as much as fat does, look at measurements instead of weight as a more accurate measure of your overall look. now, that added muscle mass does eat fat , but there is always an uncomfortable time period where you feel bigger till that fat eating machine that is your added muscle kicks in.

i'm also of the belief that you are a bit dehydrated, which can lead to some water retention, hence the tightness of the pant. drink like a fish the next few days and those pants will fit again.