Gained a pound instead of losing a pound?


So I've been exercising and eating under my calorie limit by approx.500 calories sometimes 600 unintentionally) a day.

The first few weeks I lost a pound a week which was awesome. This week when I went to weigh myself I found that i gained a pound. I'm not really sure why cause i was doing everything the same as the last few weeks. I even weighed myself again this morning (something i told myself i wouldn't do) and I was the same weight but .6 heavier than yesterday (which is whatever cause I know there are fluctuations). Why did this happen? Has this happened to anyone else? It's so disappointing. Thanks everyone :)