Why does sex make me tired??

My girlfriend is a prostitute and she is only home during 2 times a day which is around noon and at some point in the middle of the day. When we have sex at noon, it makes me really fatigued and the only desireable thing at this point is a nap. So one time i fell asleep and missed work and my boss fired me. By the way, i live in the UK so prostitution is legal. Is this normal to get extremely fatigued after sex?