Thanks for your question on Health Care Magic. I can understand your concern. You might have developed injection abscess. Flu vaccine is given intramuscular (into the muscle). In normal circumstances, mild pain is likely after flu shot. But when vaccine was not given properly, it can damage muscles, nerves and blood vessels. And this can cause local site abscess formation. This abscess gives a lot of pain. So possibility of infection site abscess formation is more in your case. Get done ultrasound examination of local site, if there is abscess then you need incision and drainage (I & D) to remove abscess. You may also need antibiotic and anti inflammatory drugs. Don t worry, you will be alright. First rule out abscess at injection site. Hope I have solved your query. I will be happy to help you further. Wish you good health . Thanks.