My last period was april5-10. i have not had sex since dec, did not use condom for 10sec. after 10sec we used the cond. i have not gone to the docs yet. what is the reason why my period hasnt come?

Further evaluation: Pregnancy should always be in the defferential diagnosis when there is amenorrhea (no period); but other causes such as hormonal imbalance, anorexia, medication etc.. Can cause that, thus you should see your doctor if amenorrhea continue; also, you may want to have a pregnancy test.

Timing is everything: While pregnancy can happen even in that first 10 seconds with so called preejaculation ("precum"), this is not likely for you if i understand the timing in your question. You would be 8 plus months pregnant and it would be obvious. I am sure you checked pregnancy tests. Other reasons for missed periods are varied and should be evaluated by a doctor. Most common is polycystic ovarian syndrome.