cheated and might be pregant .

I cheated on my boyfriend on Aug 18th. I had my period but it was verry painful on the 23rd of august . I let my boyfriend know about how I cheated and then I found out how he has been texting a girl for the past 5 months of our relationship. the beginning of September, we apologized to eachother . we fucked on labor day about 3 times. now I'm late on my period. I don't know what to do . I took a test but it says I'm negative. I told him I might be prego but he keeps bringing up the guy I cheated in him with but he is excited to start a family. we are both verry scard because I am only 17 and he is 20. any thoughts on what's going on with my period? or what I should do ? please let me know.