Always Feeling Weak After Sex

I've been a married housewife for over 10 yrs now and everytime after sex, my body becomes weak leaving me forced to lay down in bed resting.

If I have sex at night and go to sleep after, the following morning I would be generally weak. I've seen a doctor and had blood tests done and results was all good. I take multivitamins everyday but problem still persists.

My question is, what is this condition called and what can I do about it ?

Thanking you all in advance for your responses.