I was very sick 32-34 weeks (2 years ago!). Hadn't vomited at all in first or second trimester but suddenly got nausea leading to violent vomiting lasting several hours, every few days for 2 weeks. Funny thing is that I didn't feel I'll once I'd stopped vomiting, just tired. I read up about it and apparently there's a surge od hormones as you enter the third trimester which can cause nausea. It's quite rare though. I didn't worry as once the vomiting stopped I felt fine and could still eat/drink.