I had the opposite problem as the other lady. Yasmin was really bad to me, but Yaz is like a miracle. I have endometriosis and being on Yaz is the first time I can live without pain. I don't bleed anymore at all, have no cramps, and no daily pain like before. Surgery didn't even work as well for me. It did take some months for my body to finally get used to it- I'd say about 6 months until I got where I am now (in very happy land). I never had horrible problems on it in the beginning, but still had pain and some bleeding, but not much. So, it really, really depends on your body and your hormonal needs. I'd say if you're bleeding that much, it may not be a good match for you. I've probably tried 4 or 5 different oral bc's 'til I found a good match. I usually gave them a couple of months unless I was having big problems.
Oh, and I definitely weigh over 140 lbs- that's not an issue in my case.