Highly doubtful you are pregnant from that incident. The yellowish discharge can mean bacterial vaginosis, which isn't an std. Are you sure it's yellow and maybe not a drop off pee mixed in with your normal discharge? The "pimple" is suspicious, but if it is herpes you did not catch it from your one night stand. I think you should have anything abnormal on the genitals looked at within 24 hours of appearance for an accurate diagnosis. I think it's in your best interest to get cleared of std's, especially the bacterial ones, before engaging in sex again with your husband. Just as a safety percaution. Good luck!
Relax; this sounds like a very low-risk situation.
1) I'm not an expert on conception statistics, but I believe the chance is tiny.
2) Yellowish vaginal discharge could well be due to chlamydia, gonorrhea, or other STD, especially if it is not something you have experienced before. See a health care provider right away.
3) Atypical for herpes, especially an initial new infection, but possible. Intitial genital herpes can cause vaginal discharge as well as pimple-like lesions, so all the more reason to get examined promptly. The sooner you are seen, the more accurate your provider's diagnosis will be, both by direct examination and by lab testing.
4) Not to sound too glib, but they call them STDs because you have to have sex to transmit them. STDs are never transmitted to household members nonsexually.
Bottom line: Get checked right away. If you don't have (or don't want to use) your primary health care provider, a local health department STD or family planning clinic would be an excellent choice for expert, confidential care; or a local Planned Parenthood clinic.
Best wishes-- HHH, MD