5 Negative HPT's, on Yaz, 2 mo late!

I am currently on Yaz and I take it every night around the same time. About 5-6 months ago I missed my period for a whole month, took numerous HPT's, all of which came out negative. I finally went to get a blood test which also came back negative. My period returned the month after.

Now I've missed my period yet again, and I am now a little over two months late. Last month, I waited a few days after my missed period to take a test and that came out negative. I took two more each within a week of each other and so far I've taken 5 HPT's all of which are negative. I have to admit though, I've never really had a very regular period, I've always been a few days late, and like I said before, I did miss one in the months prior. But I've never been two months late! Is this normal? I thought that birth control pills were supposed to help regulate your cycle? I also thought that stress might be a factor, I just finished up the spring semester and work has been pretty hectic, but can stress really delay a period for two months? Any advice would be greatly appreciated!