As wingnut said, as long as you have unprotected sex there is a chance you are pregnant, but I presume you knew that. If you are looking to find out which days you are most likely to be fertile then a really really rough guide is that the AVERAGE ovulation time on a 28 day cycle is 14 days after you period starts. However, this means diddly squat as I am sure most of the pregnant ladies on here will tell you. Your body can release eggs whenever it wants and can change month by month, also sperm can live inside you for many days (I believe it is up to a week) just waiting for that egg to be released.
The only way to know for sure is to buy a pregnancy test, but also be aware it may be too early now for the teat to detect the pregnancy hormone so if you are panicking and need to do one now, if you get a negative result, I would keep doing them every couple of days until your period arrives.
Oh, and while you are in the pharmacy pick up a packet condoms they not only protect against pregnancy but also STI's - which your pregnancy test wont be able to detect.
Good luck.