Feeling tired before and during period

Why do I feel so exhausted and tired before starting my period, I started it 3 days late but in that time I felt as though I had a sleepy spell over me everyday, waking up exhausted and going to bed feeling even more exhausted and napping at work, really. Now I am on my period and still feel very sleepy. The other day my friend and I decided to hold off on sexual because I knocked out twice while watching a show with him. I mean, wtf???? And today I spent it running errands with my sister and most of the day I was laying down in the back seat. Mind you I had coffee with extra espresso and didn’t wake me up. Why am I so tired?