Dehydrated at 31 weeks

I just went to the ER today for feeling hot and sweaty, feeling like passing out, lightheaded, thirsty, increased heart rate. Hospital took blood, urine, fetal heart rate and EKG. They said I was a little dehydrated which I thought so since my urine was dark yellow. They gave me IV fluids and I have been drinking water all day. I felt better after leaving ER and now I am feeling bad again. Like when you get up you feel winded and lightheaded and feels like you dying in a sauna. Is this normal being 31 weeks? Baby is ok thank god. He was sluggish not moving much last night and that's when it started but now he's moving like crazy again.