What Causes Weight Gain While On Birth Control Pills?

hi. iv been taking microgynon for about 6 years now and have never had any problems with it before. however in the last month or so i have put on some weight, about 8lb. in all the time iv been taking microgynon iv never put on any weight at all. my diet hasnt changed i eat healthy and dont eat junk food. also i think the weight iv gained has gone onto my tummy, it was flat but is now a bit bigger. in the last few days iv had no appetite and have not felt hungry at all. i did bleed during my 7 day break and have started a new packet of my pill, however im getting concerned now my appetite has gone. is it possible that i am pregnant even though i bled during my break from the pill? thanks x