My Monthly

I got into a new relationship. We been together 5 months going on 6. We always use protection but in the 4th month being together we did it without protection but he always makes sure he pulls out even when he feels he is about to pre cum.  But ever since i been with him my period continues to come late. Ill even miss a month and it comes the next month. I had took two pregnancy tests and they both came out that i was NOT pregnant. But i am still concerned. My period comes regularly when it feels like than starts to scare me again when i miss it. I did it get it last month but i did get it this month. Please help me and explain why is this happening.