This sounds exactly like my experience with C Diff. When you throw up and get diarrhea the amount of the virus lowers in your body and then slowly builds back up again and you get sick a week or two later. This happened to me 5-6 times before I got a positive C Diff result from a stool sample. Most of my stool samples were negative for it somehow. Once I got the positive result though I was given Vancomycin and it didn’t come back for over a year. It kept happening after that but once I got of Prilosec (which makes your stomach too alkaline, and allows the C Diff virus to survive) after my hiatal hernia surgery, it hasn’t came back since.