There is a chance you could be.. but one of the things you need to do first is determine when you are ovualting. Generally. people with a normal 28 day cycle would ovulate around day 14 (counting your first day of your period as day 1). But, some women - like me - have irregular cycles and so it's diffcult to determine ovulation.. you can use ovulation kits, take your temps each morning, watch for fertile signs, etc. But there is no real way to know if you ovulated and had intercourse on the right days unless you get an idea of when you ovulated. There are lots of great books/internet articles that will guide you in the right direction.
I hope this helps! Good luck!