Welcome to our Forum. Congratulations to your commitment to safe, condom protected sex. As you suggest discussing past sexual relationships and STDs is a good idea as well but condom use is the most important part of personal protection against STDs.
You risk for STD from the encounter you describe is low. When condoms come off as a partner is removing themselves following sex, typically the condom as one its job and there is no risk for STD so my suggestion is that you need not worry. On the other hand, all STDs can be asymptomatic and if you have not been tested in the past, screening is a good idea. As a matter of personal protection, we recommend annual testing for the most common STDs (chlamydia/gonorrhea/trichamonas) for anyone who has had new, or two or more sex partners in the past year (we consider this to be health maintenance- we also recommend you get your blood pressure and cholesterol checked regularly).
I hope this comment is helpful. EWH
Thank you for the reassuring and honest reponse to my question.