Feel crummy just before labor???

Sooo, I'm 39+6 today and I've had a pretty great pregnancy and all week have felt great! Have been having infrequent cramping, low back pain, and BH contractions since Monday and today frequency has picked up. But also today I've felt just kinda crummy, light headed, mild headache, so tired, and weak - sitting on the couch all day (not like me at all). I've heard other women say this can be a sign of impending real labor and it's your body's way of forcing you to rest before the big marathon. Any other women on here have a similar experience?