***Please understand that I'm not trying to discount anybody's first-hand experience with HIV/AIDS***
First of all, you didn't do anything overly risky. HIV/AIDS education is unfortunately messed up beyond belief in this country, allowing some people to believe they are at equal risk of contracting HIV as other people, while others get little to no education about how the virus is transmitted under sexual conditions particular to themselves. Let's state up front: just about everybody can contract HIV under the right circumstances-- however heterosexual sex with partners who have no IV drug-use history and have fewer sexual partners is fairly low risk. HIV/AIDS cases are still proportionately confined largely to gay male populations, IV drug-users and men and their female sex partners living in very orthodox cultures where having male-to-male sexual contact is so taboo that prevention efforts are often stymied by cultural mores (this means that these ostensibly "heterosexual" men who have sex with other men have a greater chance of passing along infections like HIV to their female sex partners because they are disallowed the comfort of open communication and hence, education and healthcare). That being said, everybody should be concerned for their health and the health of their sexual partners, male, female, gay, straight, black, white, yellow, green, brown, (if one wants to think in such rigid terms) and otherwise. You should be tested for HIV with some regularity if you are sexually active, but unless you have reasonable suspicion that you were exposed, you shouldn't worry yourself sick about a one-time lapse in condom use. If every "heterosexual" were at risk the same as a man who has sex with men (either those who do it under discreet conditions or those professed as gay or bisexual), then the comparatively lower use of condoms in heterosexual sex contacts as compared with gay male sex contacts would mean that the HIV rate would be substantially higher in the supposedly-strictly-heterosexual community. This just isn't the case----- yet.
However, things change-- already HIV/AIDS is afflicting communities of color much harder than their hetero-euro-descendant counterparts, for the same reason why I stated above-- HIV/AIDS education and healthcare are culturally inaccessible in many cases (to everybody, but especially men who have sex with men discreetly but lead "heterosexual" lives)-- throw in identity politics and it just becomes a mess nearly impossible to sort through. So, women of color (hardest hit are black women, but also latinas) are being exposed to HIV, often times because of socio-economic status disadvantages. This is a large bridge into the "heterosexual" world, but depending on who you are, who you have sex with, your chances of contracting HIV are very different from the poster-boy images of the early 1980's.
In order for a male to contract HIV from a female during sex, he would pretty much have to have blood-to-blood contact with an infected partner (that is, she's bleeding, either from a cut/wound or menstrual cycle and the man has a opening/wound/cut that would allow the infected blood exposure to unprotected tissues), or have a wound/cut/whatever somewhere that would be inserted into an area containing virus carrying fluid (such as a cut/sore on the penis during vaginal intercourse).
And for the record, oral sex has a very very low rate of HIV transmission and requires sores/cuts in the mouth/esophagus and infected fluids to be introduced into those cuts. Saliva is inhospitable and does not carry HIV because of enzymes that kill the virus, stomach acid destroys the virus.
I don't know your sexual practices, but if you have sex primarily with women and only have the occasional "oops I forgot the condom" slip-up with no other glaring concerns from the encounter (e.g., she had cuts/sores, or said she was HIV positive) then go get tested, but don't stress out too much about it.