Question re stomach bug and immunity...more inside!?

About 3-4 weeks ago I had a really bad stomach bug, (vomiting and diarrohoea, both really badly). I'm recovered now, but I've been hanging out with my friend today who had a bad bug a few days ago (same sort of thing) and is in the process of recovering. He says I'm immune from catching bug again off him after such a short time, but I'm not so sure. Can anyone shed any light? I#m scared!