There is always a chance you can get pregnant every time you have sex, protected or not.
The only way you will know for sure is if you wait to see if you missed your period, and then take a pregnancy test.
There is "plan b" just to be sure, but you have to get it as a rx since you are under 18.
If you are going to continue having sex you should get on birth control and he should wear condoms every time.