Anytime you have sex you take a chance. No one can tell you the exact probability because there are so many factors to consider.
If you are this worried then you really shouldn't be having sex. But if you are going to be sexually active then go on the pill/patch/ shot/ something, use condoms and a spermicide. Maybe that will help your worrying.
Just try to calm down because if you are pregnant then you are pregnant, and if you aren't, you aren't. What is done is done. But if you used condoms then I think the percentage of effectiveness is 80% or something like that. You will most likely be fine.