I feel/look thin right before my period. but during and even after i feel/look fat. why is this? it's causing me to be very upset.

Fluid retention?: I understand your frustration. The body changes before, during and after menstruation (period). It is actually a natural change. But at the same time the change is not that drastic per say. If you want you could measure your weight before during and after the period. You will likely see maybe a couple pounds in weight change due to fluid shifts in your body. But this is normal, your OK.