Strange weight changes...what could be causing these?

I have sudden, very strange weight changes. I could weigh myself, wait four hours, and have gained five pounds. I could weigh myself the next day, and weigh seven less. This summer, I have been dieting and excercising, and I've lost over twenty pounds. However, now I cannot go any more below my current weight, regardless of making my diet and excercise more and more strict and frequent. What could be causing this? And how can I continue to lose weight?