What Causes Weight Gain After Stopping Birth Control Pills?

I have taken oxyelite before. Reached my goal weight and just recently got off birth control pills. I'm quickly gaining Weight back...about 5 lbs in 3 weeks. Not typical for me. I haven't changed my eating habits or exercises. Is this normal and if I take oxyelite and stop will my weight keep going up and down. Am I messing something up in my body? ?