Dear friend,
The technique by which injection was given to you was partially wrong.
Intramuscular injection is suppose to be given inside a muscle. In certain occasions nurse/doctor give it partially in fatty tissue leading to formation of lump and pain.
Initially you can give ice fomentation to relieve local pain. But if it's not relieved or pain increases then you need to visit a general practitioner for corticosteroid intralesional injection.
Next Steps
ice fomentation.
if not found useful then visit a good doctor for further evaluation and treatment.
Health Tips
kindly visit an experienced health care provider next time to avoid such negligence.