My mind would have immediately gone to an STD or infection, since you've mentioned that you are on the pill, I don't think you've mentioned condoms.
Painful Burning during sex
There are several possible conditions that might be causing your symptoms. Some of them follow:
1. Yeast infection
The typical symptoms are:
redness, itching and sometimes burning during urination
pain during intercourse
white, cheesy discharge (usually but not always present)
Yeast infections can usually be treated with a course of over-the-counter topical antifungal medication.
2. Contact dermatitis
You might have this condition if you have recently come into contact for the first time with the following:
a cream or jelly spermicide
a feminine hygiene deodorant or douche
a scented product such as sanitary napkins or tampons
latex condoms
colored or scented toilet tissue
certain soaps, detergents or fabric softeners
A short investigation should do the trick, and all you need to do to end the symptoms is discontinue contact with the irritant.
3. Lack of lubrication
Your discomfort might be caused by something simple, like needing more lubricant during intercourse. Try one of the artificial lubricants on the market such as K-Y Jelly or Astroglide.
4. Urinary tract irritation or infection
Sometimes women who are having frequent sex get a UTI usually referred to as "honeymoon cystitis." It is an infection or irritation of the urethra or the bladder that can cause painful burning when you urinate. Emptying the bladder before and after sex, and drinking lots of fluids, can help stave off this irritation.
More on Burning during sex
TeenHealthFX is concerned about your burning sensation after sex, and the fact that you have unprotected sex with your boyfriend. We suggest that you go to see your doctor as soon as possible. Many different things, including an irritation, a sexually transmitted disease, such as Chlamydia or Genital Herpes could cause this burning sensation. If not taken care of properly, this burning can cause damage to yourself and your boyfriend.
Regardless of what the burning is being caused by, TeenHealthFX thinks it is a good idea for you not have sex until you check with your doctor. If this is an STD, it is important that you don’t spread it to your partner. Before you see your doctor think about any other possible pain or discomfort you may be having such as vaginal discharge or abdominal pain, this will help the doctor to determine more accurately what might be causing you to feel this persistent burning.
(I beleive in the health forum, you also mentioned that you were having abdominal pain, this might be related to that)
Get to your dcvtor sooner rather than later.