It's often due to low BP. I get it right the way through pregnancy because my BP is always at the low end. My low BP also causes severe headaches and nausea. It is an annoying symptom but I figure I'd rather put up with this than have high BP and pre-e.

Definitely mention it to your care provider but I wouldn't be too concerned in the meantime. Like everyone says, eat and drink regularly and stand up slowly. Good luck!