Hi , My 5 Year Old Son Has Small Bruises

Hi , My 5 year old son has small bruises on his legs an pain when he stands on his left leg he went to the doctor today because of a small blister because his sister has a skin infection almost like infentigo an was put on a antibiotic called bactrom I don't know if he is really hurting or is playing it a little would like to know what you think