3rd trimester, woke up queasy?

30w4d. I've never had a problem with morning sickness. I woke up early this morning (around 4am) incredibly nauseous. I was literally gagging and thought I was going to throw up. I propped myself up in bed with pillows and waited it out, eventually I fell back asleep. I woke up again around 9 and felt ok, but now it's 11:30 and I'm feeling really nauseous again. I feel like I could throw up at any moment. What could be going on?