Constipation & Menstrual Weight Gain?!

Yesterday morning I weighed myself and was 123.0, I ate very little yesterday..like maybe 600 calories at most because I had horrible cramps and wasn't hungry. I went to the gym and did some cardio too. But this morning I weighed 124.4. I don't how I gained almost a pound in a half considering I was at 300 net calories for the day. I figured it was my period..and probably because I haven't gone to the bathroom in about 3 days. Do you think it was just that?