Sudden weight gain

I just had a lot of large changes in my life (moved across the country from school to home a week ago) and I'm starting to try to get in shape again. The trouble is, I weighed myself 3 days ago and I was 140.4....and the next day, at about the same time of day after eating about the same amount, I weighed 148.8. Weighed myself again today and I was around 148 again.

I don't understand. This past week, I started exercising and eating healthier - around 1800-2000 calories a day, which is about 500-700 less than I was eating before moving back. I've been working out for 20-40 min every day (elliptical, running, yoga, stretching), and it just doesn't make sense to me that I've gained 8 lbs in a day and it wasn't just a fluke. I know I haven't been working out that long or eating healthy for that long, but It's really disheartening right at the start to gain a significant amount of weight for no apparent reason when I'm trying to maintain, if not lose weight. I've always been really active and fairly healthy (between 125-135) until last semester when I had an injury that kept me from being active, so this "getting back in shape" thing is new to me.

Any suggestions or advice? What am I doing wrong?