Hi I Haven't Been Feeling Good For The Past 3

Hi I haven't been feeling good for the past 3 days. Usually my stomachs really upset and I won't feel like eating. My upset stomach will go away then I'll feel like I can eat everything! Then after eating I get a upset stomach again and feel like puking. I have excessive gas and salivation. I went to the hospital before and they test me for ulcers and other stuff but It came up negative...What should I do?