My Wife As Been Experiencing Constant Nausea. At First We

My wife as been experiencing constant nausea. At first we assumed she may be pregnant. But a test and her period let us know that was not the case. It can be in the morning, noon or night. Her stomach doesn't necessarily hurt except from heaving. Just nauseous. The weird thing is she gets nauseous and goes to throw up but just burps mostly. Constant nausea and burping. This has been happening for a couple weeks now. Thank you!!