What you're describing doesn't sound like nervous tension, vaginal tightness or insufficient lubrication - it sounds more like a pelvic disorder (perhaps an infection) as opposed to a vaginal one, as the problem only arises when your boyfriend attempts deep penetration. There are a number of possible causes that jump to mind, including:
•Pelvic inflammatory disease. This is an infection of one or more organs, including the uterus, cervix, and fallopian tubes. PID is caused by a bacteria or other organism entering the vagina and traveling upwards, to the cervix and further. You should visit your doctor at once to rule out PID. Caught early, it can often be treated with antibiotics but left untreated, it can cause serious medical problems.
•Endometriosis. This condition occurs when cells from the lining of the womb form implants that start to grow outside the uterus, generally in the pelvic region.
•Ovarian cysts. These are fluid-filled sacs that sit on the ovaries. If they grow large, they can cause feelings of pressure, fullness or even pain.
•Fibroids. These are non-cancerous growths found in the muscle of the uterus. Sometimes, if there are lots of them or they grow very large, they need to be surgically removed.
•Tears in the ligaments that support the uterus. These may be caused by childbirth, a badly performed abortion or violent sex.
•A condition affecting another organ in your pelvis, such as your bladder, kidneys or bowel. Strange though this may sound, even irritable bowel syndrome can cause pain during sex.
It's also possible that you have none of the above conditions and that your problem is a passing one, caused by something perfectly trivial. You really must take this issue to your doctor and he or she will run the necessary tests to get to the bottom of the problem. Please take action soon - it's important that you don't drift on this one.