I am 24 years old .I had unprotected sex on 11 th day of menstrual cycle my periods on 13 and sex happened on 23 on 26 and 27 I had a stomach pain and back pain .I want to know that if there is a chance for pegnancy or not

You had unprotected sex... thus the possibility to have gotten pregnant would ALWAYS exist.

Even WITH protection its still always a possibility. You will have to wait and take a pregnancy test to be sure.

Remember no form of birth control is 100% effective except abstinance.

We have several prominent members here who have gotten pregnant despite properly using 3 separate forms of birth control at the same time.

Pregnancy can happen. Since you had continued unprotected sex I will assume, since you did not state otherwise that you are trying to get pregnant. If you time your most fertile times it will work better. But yes there is a chance now.

At this point you will just have to wait and see. Good Luck