You might also want them to check your blood pressure, in case it is running low. This can cause you to faint when raising from a sitting position.
I agree...talk to your doctor about it. Your body is working extra hard to get nutrients and oxygen to your baby. Now is a time where you need a lot more sleep to gain energy throughout your day. Good luck...hope you feel better soon.
You need to tell your doctor, most likely they'll want to check you for anemia (iron deficiency) and check and make sure there's not a clinical reason that your blood pressure is dropping drastically. It can be normal, but excessive fainting and weakness needs to be checked out immediately. I wouldn't worry, most likely it's just something about your body having trouble adjusting to this part of pregnancy, but you need to be seen to find out why, because it CAN be an indicator of pregnancy-related complications. The good news is, these types of things usually only affect the mom, not the baby, and disappear after delivery....but don't worry too much it's likely nothing. Good luck :)