I had protected sex with a girl who had had unprotected sex with others in the month prior, and I am concerned about HIV risk.

The problem is that the condom I chose to use was a baggy one, the Trojan Ecstasy Ultra ribbed (feel free to look it up to see what I'm talking about). It was baggy all the way down my shaft, allowing for extra movement of my penis. It is a very wide condom. It had no reservoir tip. I'm concerned because at some points when I took my penis out of her in between having sex, the condom had bunched up and slightly hung over my penis. I pulled it back so that the tip of the condom was at my penis tip. There was ALWAYS condom covering my head and tip and part of shaft during penetration, and ring was always on a part of the shaft, but I am concerned about vaginal fluids from the penetration going from her to my uncovered part of shaft, and then when I pulled the condom back down as it hung over my penis I may have introduced vaginal fluids from shaft to the tip of the condom and then to my urethra? Or would that be no risk since it is analogous to mutual masturbation / secretions in the outside environment? I'm concerned that her vaginal fluids got into the loose fitting condom (my penis tip directly touched her vagina but am concerned fluids got inside baggy condom and to my tip). I'm also concerned that if she had had unprotected sex with others in the month before me she could be in the acute part of HIV and more likely to transmit (If she has HIV, which I know to begin with may be a low chance).

Thanks for taking the time to respond.

You are focused almost entirely on the condom and whether it protected you completely. It probably did -- I'll get into that in a minute. But you should also think about two other factors:  How likely is it your partner had HIV? And if she did, what were the transmission risks, without a condom? The answers are 1) very unlikely (probably under 1 chance in a thousand) and 2) also very unlikely -- the average transmission risk from unprotected vaginal sex, if the female partner has HIV, is around once for every 2,500 exposures. That's one reason many spouses of HIV infected people never catch it themselves (which perhaps you didn't know). In other words, in the US and other industrialized countries, HIV rmains rare in even the most sexually active heterosexual singles.

For these reasons, even if you hadn't used a condom, I would estimate the chance you caught HIV at less than 1 chance in a couple million, and wouldn't even recommend HIV testing after only a single exposure like this. Don't get me wrong:  I'm not recommending casual sexual encounters without condoms. Even though the risk of HIV is miniscule, it isn't zero and why press your luck? And of course there are other STDs to be concerned about.

Now to your specific questions:  You are correct that as long as the head of your penis and the urethral opening were covered, this would be considered safe sex in regard to HIV, as well as for other STDs transmitted primarily through genital fluids, i.e. gonorrhea, chlamydia, and hepatitis B. There is somewhat higher risk of those transmitted skin to skin, like herpes, HPV, and syphilis. However, syphilis is even rarer than HIV in partners like yours; HPV is pretty normal and universal anyway (you probably already are infected and there is little increased risk from any one exposure); and herpes also is uncommon in this situation.

So I don't advise testing for any STDs unless or until you develop symptoms. However, if you would like the reassurance of negative test results -- even though the risk is extremely low -- you could have a urine test for gonorrhea and chlamydia any time more than 3-4 days after the event, and blood tests for syphilis and HIV at 6 weeks.

And of course I don't need to tell you to purchase your own, properly fitting condoms!

I hope this information is helpful. Best wishes and stay safe--   HHH, MD