What Causes Weight Gain While On Birth Control Pills?

Ive been on my third BCP now for 4 months and out of no where I gained a few pounds all in my belly (without changing diet or exercise plan) and now that I did change it (eating healthier and working out 3 times a week) it will not go away. I had bloating issues with my last pill for a couple months, how do I know if the new pill is the cause and if it will go away?