missed periods

me and my boiifriend had protected sex and we didn't go all the wayy the first couple times and each time blood came out. After tht I don't have a period 4 awhile and then I got it. after that we went all the way and the condom broke and it went in once and out like a second or 2 later he felt sumthin wrong. He said tht nothing came out. It has been a few months since my last period and does having sex mess with your cycle? I took a pregnency test and it came out negative and I haven't been eating very much because I have not been hungary and have lost weight. My body is really trippin out I think. I am also under a lot of stress. So what could possibly be why I have not had a period in 2 or 3 months? Thanxx