39 weeks Pregnant

Ive been having on and off cramping for about 4 days. Now today i wake up and i feel so tired. I keep falling in and out of sleep. I dont want to get out of bed but i have too(i have 2 boys). Im also very nauseous and weak. Is this normal? My first 2 were preterm babies so i never made it this far. Im not sure how im suppose to feel