Breakthrough bleeding on birth control?

I have been taking Yasmin for two years now with only a few problems. Since I started taking the pill, I have had breakthrough bleeding when I'm stressed (I'm not sure if thats what it really is) Its like a full blown period but its always a week or two before my normal period, and then I get my normal period as well. I started taking the Alli weight loss aid two months ago and have been losing about two pounds per week and have been a little stressed. The first week I started taking Alli I had an early period and then my normal period a week later. Today I am having slight cramps that feel kind of like I may have a period but it is not due for another two weeks. What do you think? is it the Alli, the stress, the weight loss or is Yasmin just not working for me anymore? I haven't been able to find ANYTHING online about Alli affecting birth control or periods.