Open Question: Please help in my sexual health?

I am 28 year old man where every i had sex with my bf i feel vey sleepy ,i feel as no enegy in my body My weight is 70 Height 5.8 Same height and weight is my gf ,but she does feel it as i feel it after sex Can anyboy tell the reason for this in general are medically Please help me i need all ur help friends And one more this i would like to ask i am on antidepressant from 8 year