What changes occur to a womens body when sexually active?

Nothing specific. As you age and you hormone levels change, there will be noticeable changes in your body - unless you conceive. Teh sexual activity itself will not induce any specific changes in your body beyond what one might expect from the actual sexual activity (some tearing of the hymen can occur, you risk for urinary bladder infections can increase, and of course the risk for std's is present).
Physiologic changes. When a female becomes sexually stimulated lots of changes occur. Blood flow increases to the pelvis causing the vagina, labia and clitoris to become engorged - slightly swollen. Blood pressure, heart rate and respirations increases. Glands in the vagina release fluid which causes lubrication.