3 year old was on antibiotics after 2 weeks she is having itching on hr back and sudden appearance of red bums tht looks like bed bug bite. What is it?

Many possibilities. But the most likely is that she is experiencing an allergic reaction to the medication. Recommend that she be seen by the doctor who prescribed the antibiotics, both to document the reaction for her future care, as well as to make certain that nothing else is going on. Good luck.