Hi there...I can understand your concern. By back I am guessing you got injections to your buttocks. For the buttocks usually intramuscular injections are given to the gluteal muscle. Intramuscular injections are usually very painful. The pain can persist for weeks. But sometimes it can lead to the formation of injection abscess or antibioma. Only through clinical examination it can diagnosed. So kindly consult a general surgeon for further evaluation.