1. You're dry.
The sex will be painful when you're not wet down there. (And yes, you can still be pretty dry even if you're aroused.) Solution: Keep up the foreplay, relax, or use water-based lube.
2. You're allergic to semen.
This is a rare case; you need to be tested by your doctor to know for sure if you have it. Some symptoms include the swelling of the vagina or vulva when it comes in contact with semen, redness, itching, and a burning sensation.
3. You have ingrown hairs.
No thanks to your bikini wax, which can give you pimples around that area. Not much you can do about it in the heat of the moment, but in the future you might want to make sure you keep that area clean—free from oil and sweat that cause bacteria.
Which is to say, DON'T pop the pimple; you might just make more zits come out. Apply pimple-killing product like tea tree oil instead and make sure to exfoliate that sensitive area once a week to keep pimples from popping up.
4. Your partner's too big.
If he's in too deep that he's hitting your cervix, you're bound to get hurt. Tell your partner to slow down or be more gentle, or go on top so you can be in control. Lube can also help.
5. The sex position you're doing isn't for you and your partner.
The moves you should and shouldn't be doing depend on the curve of your guy's penis. For instance, if his penis curves upward, missionary or any position where you're facing him will be great. Doggy-style? Yeah, not so much.
6. You have an STD/STI/pelvic disease.
Herpes, chlamydia, pelvic inflammatory disease—they all cause pain during sex. See your doctor for treatment, practice safe sex, and get checked regularly especially if you have multiple partners.
7. You have a vaginal infection.
Like a yeast or bacterial infection. See your doctor to check what's up and to find out how to treat it.
8. You have a uterine problem.
These include fibroids (benign lumps that grow on your uterus) and endometriosis (a condition where cells from the lining of your uterus grow in other areas of your body and stay inside your body). Both can be treated.
9. You're nervous or afraid to have sex.
You can expect some discomfort during sex whenever you're not relaxed. You're either dry (see #1) or you're afraid of getting hurt through sex because your vaginal muscles squeeze or spasm when something is entering you (it's called vaginismus). What can you do? Try these exercises to help control and relax those muscles.
10. You just gave birth or had surgery.
Your vaginal tissues can tear if the delivery was forceful, or your vagina can get an injury. Give it a few months' time to repair.
Follow Stephanie on Twitter.