Lack of arousal.
“For a young woman,” says Dr. Hutcherson, “the number one cause of pain during intercourse is lack of arousal, most often because she’s not getting enough foreplay from her partner.” When you haven’t had enough time to become fully aroused before intercourse, the vagina isn’t lubricated, causing uncomfortable heat and friction as it is penetrated. Treatment is easy and fun: Just tell your partner you need more time and attention lavished on you to get completely in the mood.

Infection.
You could have a yeast or urinary tract infection, both easy to treat and not dangerous if taken care of right away. “A month ago, sex started to be really uncomfortable for me,” says Lesley, 24. “I assumed that the guy I had just started seeing was awful in bed. Luckily, before dumping him for lack of chemistry, I visited my doctor and found out that I had a severe yeast infection with none of the usual [white, clumpy] discharge signs.” A doctor will prescribe an over-the-counter or prescription treatment and, if you’re anything like Lesley, the sex after the infection clears should be back to “amazing.”