Dizziness?!

I started feeling dizzy on and off since Tuesday. Yesterday it got bad and I had to leave work early and go to the doctor. The doctor said that I must not be drinking enough and the iron supplement that I was on was causing me to have bowel movements thus making me dehydrated. I don't really think that's what causing it since I'm very careful with how much I've been drinking and making sure I drink enough. Now today I've been dizzy all day, I tried taking the medicine the doctor prescribed me and it doesn't help. I don't think that I'm dehydrated at this point and was wondering if anyone else has had issues with constant dizziness in third trimester and if so what did you do to help and at what point should I go back to the doctor. I've been laying down all day and any time I try and get up to move the dizziness it's very over bearing.