Foxy is right. You need to get yourself on some kind of contreceptive, or buy condoms and a spermicide to protect yourself - not only should you worry about pregnancy, but about std's as well!
If you say you are about 3/4 through your cycle, do you mean you are about to get your period in a few days? If so, then of course there is always a chance of getting pregnant, but in general, if you are close to your period coming, then I might say that you are ok and not pregnant.
The most fertile days for a woman (and this is based on a 28 day cycle) is from cd10 (cd1 being the first day of your period) to cd16. After you have ovulated, the chances of becoming pregnant is slim.
Like foxy said - go and get the morning after pill, and learn from this experience.
If you have more questions, there are also local numbers that you can call, or you can ask them in here, and people will try their best to help you.
Good luck.