I think if you're consistently staying at 1500, you're probably good for now, calorie wise.
Consider this - weight loss is NOT a line. It's a bunch of bumpy fluctuations all over the place which, if you do everything right, trends downward. We have a great "why the scale fluctuates" thread here:
http://www.3fatchicks.com/forum/showthread.php?t=115826
Sometimes when you start a new exercise regimen, you hold onto water while losing fat. In this case, you stopping exercise would cause that water-weight to drop as the swelling in your muscles went down. This is normal and a necessary part of your muscles getting stronger.
Hormones are another option...depending where you are in your cycle, your weight can go up and down. It doesn't have to be bloating due to TOM or PMS. My personal trend is that I bloat up 2 weeks before my period, drop some the week before, bloat up again during, drop the week after. It's one week on, one week off. Since I'm aware of it, I don't worry too much about it.
I can guarantee that the reason you dropped weight is because of the work you did for the three weeks, not the off-plan nature of the last few days. Our work doesn't always immediately show on the scale, but if you keep working, it will come off eventually.