There are a couple of possibilities here -
Sex can often cause micro-tears in the vaginal mucosa (basically the inside of the vagina), labia, and near the anus that can be quite uncomfortable.. it can occur more frequently when you haven't had intercourse in some time. You might even notice burning with urination because the urine, which is acidic, can touch the cuts and cause pain. Often within a day or two of this happening, labial swelling and redness along with pain is very common. I would recommend increasing lubrication (KY jelly works well) during intercourse to decrease the occurrence of this. In the meanwhile, I would avoid intercourse for at least a week or two to allow these cuts to heal. Occasionally, I will offer my patients a short term course of vaginal estrogen cream to help the vaginal 'skin' heal faster.
Another possibility includes a yeast infection. Yeast infections typically occur inside the vagina but often can also occur on the outside. The skin near this area can get raw and burn/itch pretty intensely. Over the counter yeast creams like monistat is a great first step; however, if it doesn't improve, I would ask your doctor for a prescription of diflucan - this medicine is more powerful and tends to kill the yeast quite well. However, because this all occurred after intercourse, I would definitely lean more towards microtears than infection.
While I cannot 100% say it is not an STD without seeing it myself, it does not sound typical of an STD.. Herpes have blisters and hurt, Genital warts and sympilis have 'bumps' but don't hurt... Gonorrhea/chlamydia cause discharge from the cervix. Your symptoms do not match up with any of these common STDs.... and although symptoms can arise quickly, most often it takes days to weeks for any symptoms to come up. Again, this does NOT sound like an STD.
If you do develop blisters in the area, I would have a doctor 'swab' the area to look for herpes... but I doubt this would happen as this is most consistent with 'trauma' from intercourse.
The diarrhea I think is unrelated to any of this (diarrhea does not accompany STDs or vaginal/labial tears from intercourse.)
If you need any clarification or have any other questions please feel free to ask. If you are satisfied with my response please click 'ACCEPT'; bonuses and/or FEEDBACK are most welcome. Thank you!