Help Please

AnnaJensen
Okay, so I'm 16 and I've been getting my period since I was like 11. I thought that I was getting my periods monthly and that everything was all synced up... Well it's not. 
One year ago, for like a good year- year and a half I got a period every month. 
After the first time I had sex, I didn't get a period for three months. I thought I was pregnant, but I took like four pregnancy tests during different times and they all came out negative. 
The chances of me getting pregnant is low as well since we did use a condom, we did use spermicide, and we did check using multiple different ways if there was a hole. We also cleaned up afterwards quite a lot, but I do know, there is still a chance. 
Anyways, I am not pregnant. I have experienced no symptoms and the doctor has done tests. I received my period two months ago. 
One month ago I had just started birth control (haven't had my period since) and it's that week where you are supposed to get your period. I have not gotten it yet. 
Is there something wrong with me? Am I supposedly unhealthy or like infertile? Why don't I get my periods monthly like I'm supposed to? 
All of these girls freak out after missing it by like a day and I'm just like... Counting months. 
Someone plz help me, explain to me why I'm not getting my periods. It can't be stress either. Ive been on break for most of the time and have been relaxing.