Hallow Dear, I have understood your problem. Such incidence has been observed in many cases if the condom is not worn properly. When the condom slips inside the vagina, there is great risk of pregnancy as the semen leaks into the vagina. Moreover, you have noticed the presence of semen inside your vagina. The best option for you now to avoid pregnancy is to take some emergency contraceptive pill like ipill or so within 72 hours of the intercourse. This will give you protection against the pregnancy. Probably due to emergency contraceptive, your periods may be delayed be few days. If they are delayed by more than 7 days, then get your urine examined for pregnancy test. If you are pregnant, you can terminate the pregnancy by medicines up to 9 weeks of pregnancy. I hope this has solved all your doubts.