Why is it that I bleed during sex only

Hi... you haven't given very much information so I will do the best I can to help.

I presume that you are female.It could be that you are about to start your period, or that you had not quite finished a period.

Depending on your age,sometimes the vagina wall can become thin, due to hormone changes,it may be that when you have intercourse a tiny part of the skin within the vagina becomes irritated and there for bleeds slightly.

There is no mention of any other symptoms:discharge, itching, abdominal pain, etc, however, I do think you should visit your doctor, explain your problem,he/she can examin you and give you a correct diagnoses to put your mind at ease.