Totally Confused!

Okay, I am 26 years old and I have never had a normal monthly period... Usually I have them 5 times a year but they always fall on the same months each year. The thing is that I am now concerned because I didn't have a period at the end of July and august which I normally have. On July 25th I had sex for the first time and should have started within 5 days after that. Instead 8/9 days later I spotted and that was it. Since I have experienced a lot of pregnancy symptoms all of which can be considered pms, but have still not had my period. I have taken several tests and all have come out negative. However, yesterday while I was driving home from work I began having this weird sensation in my lower abdomen. It was like that ticklish feeling you get when going down a big roller coster hill. It was weird and paired up with all the other symptoms I am begining to wonder if I should make an apointment. This is just crazy to me because I asumed it was just typical worry because this was my first sexual encounter, and I have never even been to a gyno. I am unsure of what to do. Any advice out there?