I believe what you've encountered is called a "weight loss plateau".
something you may not have thought of is what you drink (or you may have, I'm not sure). Juices and soda are high in sugar and can help you put on weight very easily. So something that helps every diet is sticking to water and occasionally diet soda. water is better for your body anyway.
For better tips I found this website:
Weight Loss Center: How To Get Past a Weight Loss Plateau
and there are plenty more if you google "weight loss plateau"
hope this helps!