Hello , i have been feeling somewhat faint during intercourse almost always at the end when i feel i will ejaculate. i get lightheaded and my vision goes a bit darker. also if i masturbate on my own i feel fine, and if i exercise i also feel fine?

What type of exercise do you do then you feel fine. Are you stressing your heart with running or doing stretching or light walking? Do you have a neck problem or have the same problem extending your head? See a doctor

It appears you may have vasovagal ejaculation syncope (fainting), which has been reported in the journal Heart in 2009. The parasympathetic nervous system provides the relaxation and proper vasodilation needed for a successful erection. However that vasodilation can also cause one’s blood pressure to become too low, resulting in fainting. Talk with your PCP for more information about the problem.

Agree with Dr. Ridao. Make sure you are well hydrated before you engage in sex. Try drinking a glass or two of water about half hour before the time of sex. Wish you good health!