Constantly light headed.. ?

I'm trying to diagnose myself. Anyone else find out why they were so light headed in pregnancy?

(I have an appt with one of my Drs on Tues so I'm bringing it up then)

This is my 3rd baby, but I don't really remember being this light headed. I see stars a lot when I stand up too fast. I feel generally tired. I know I have relatively lower blood pressure, but I think that's also just a symptom or cause of something else. I'm trying to figure out if it's low iron or just something else off.

I'm 26 weeks & it seems to only have gotten worse. It just makes me so nervous.