Why dose. It burn when my boyfriend cum in me?

Infection, allergy. This could represent an infection from yeast or trichmonas or a sensitivity to friction. A gyn exam for culture is indicated. The use of a latex condom is important as added protection for birth control and prevention of stds.
Not normal . First, you should schedule a visit and get tested for infection. Yeast and other vainal infections can cause vaginal burning and intercourse will make this worse. If the tests are all normal then try using a vaginal lubricant like KY or astroglide to increase the lubrication of the vagina and reduce friction.