weight gain?!

Hi-

I am in need of some support/ advice.

I run and practice bikram yoga. I am eating 1200 calories, plus most of my exercise calories. I had lost 5 pounds over the last 21 days, but then, this week, my scale said I gained back 3 pounds. I am only weighing myself once a week. I have not gone over my calories, and not exercised any less this week. I am feeling super bummed about this- and am wondering if this could be water weight gain (I am close to my period...) or other ideas why I have gained weight back. I am feeling so bummed out, as I have been working really hard at both exercise and diet. Also, ideas to get back on track would be helpful, too.

Thanks in advance!