Why do I get like this? (Extreme lethargy!)

I’m having a bad day today, I wasn’t too bad this morning but this afternoon I feel so extremely fatigued and lethargic I can barely walk! Or I’ll fall over! My whole body hurts and all I can do is lye down! Which is difficult when I have an 8 month and a 2 year old! Iv had to get my father in law to take my eldest and Iv put my youngest in her cot next to me so I can lye on the bed. This happens to me randomly, it used to be once a week but now it’s only like once a month. I don’t understand why?