What Could Be The Reason For Delayed Periods?

Hi. I'm about 12 days late for my period. I havent had sexual intercourse but me and my boyfriend tried to stick it in but it wouldnt fit. It was a one time try but he didnt penetrate. That was a day before my period was expected to come)the 19th of October). My last period was on September 12 and my periods come between 35 to as late as 38 days. I was very stressed the week my period was supposed to come on and I even took a pregnancy test a week after the 19th. It came out negative. I went to the doctor Thursday and they gave me a test and it came out negative. Im scheduled for an other appointment this wed. Im experiencing period like cramps, but no period. What are the chances that i'm pregnant? or is there something else wrong with me?