Welcome to health care magic,
As per your description,It could be cystic acne
or furuncle etc.
It has nothing to do with stomach upset
or loose motions.
You must take antibiotics ( cephalosporins preferred) with antihistamibes and antiinflammatory to get rid off that.
You wash them with savlon and water.
Only 3 days course of antibiotics Would be sufficient.Consult dermatologist
for further advise.