I think you should stop worrying if only because it gets us nowhere. The likelihood of your having impregnated that lady are very remote indeed.
The above is an ideal plan. This can be implemented if both are in ideal relationship and cool headed.
Everyone knows that unfortunately condoms are not 100% effective.
What is irrational is having random sex with someone without being prepared for the risks.
There is always a risk of pregnancy even with other birth control methods, and not just pregnancy, but also std's and sti's.
That's why you are really supposed to discuss preventative plans with the people you have one night stands with before you have one night stands with them....
I had casual sex with people without discussing things because I'm a woman and I knew I was on birth control and would do everything in my power to prevent pregnancy at all costs, but unfortunately you are a man and do not have that control over her, I see this happening all the time and there are a lot of women out there that purposely get guys to impregenate them. I don't know why they do it, but it happens a lot. As a man, you have to be very cautious because there are a lot of desperate girls that do that these days.
Don't have sex with people you dont want to impregenate or get std's from unless you have first talked to them about that big risk factor, "what if" and if you were smart you would also get them to agree to going to a sexual health clinic with you to get tested before you randomly **** them, so you can know for sure they are telling the truth about being clean. If all you want them for is sex and vice versa, they'll be more than happy to get checked for diseases and infections with you before you guys get together, because chances are they also sleep around alot and care about not catching something.
Condoms are less effective than most other types of birth control methods. Next time choose someone who is on something that is much stronger like Depo Provera and who isn't against taking the morning after pill or even more extreme preventative measures if pregnancy ever happens.
In the meantime, I'd say you have a reason to be worried, so hopefully you still have her number and can talk to her to make sure she's doing something to take preventative measures against pregnancy. She should have taken a morning after pill even though you used a condom, because everyone knows condoms are not the most effective things...
Who knows, maybe she has her tubes tied and can't get pregnant so it didn't bother her or maybe she was just as impulsive as you and neither of you really thought things through.