My son (10 yrs) always has bumps that look like flea/mosquito bites all over his legs and some on his waist area every spring/summer, His doctor says its allergies to trees and grass. Those have healed, but I noticed him itching his bottom and checked and found that he has some bumps like mosquito bites and some that are like welts (similar but not the same). Tonight one looked like it had something coming out and I put my finger near it to check and it oozed some puss and a little blood. It seems like they are getting better and then get worse.
HI...this seems like an infected allergic rash. The initial insult should have been the allergy as your doctor has pointed out rightly. The scratching of the lesions might have led to secondary infection. Pus oozing it means...it is bacterial infection and required antibiotics either topical or systemic. I suggest you get your kid seen by your pediatrician and get an antibiotic prescription. Regards -