Henoch-Schonlein Purpura- (HSP) symptom questions

My 6 year old son was diagnosed with Henoch-Schonlein Purpura aka HSP 3 weeks ago. We have been fairly successful treating him with ibuprofen and used prednisone once. The purpura rash that comes and goes just came back 2 days ago. He had gone 3 days without pain and ibuprofen. Last night he woke and vomited several times. After the 3rd bout, I gave him the ibuprofen. He has been fine since that was 8 hours ago, but just had a bout of diarrhea. I am trying to figure out if the vomiting and diarrhea are from the HSP or if he picked up a stomach bug somewhere? How can I tell?  No fever or pain and otherwise acting like a typical 6 yr old that is not sick.

If this is from the HSP and it has moved into his bowels, do I have other concerns? Last week his urine test showed no protein. We don't go back for 2 more weeks.