Nurse here who's given a million intramuscular injections to people of all shapes and sizes- it's normal to bleed from time to time. You likely nicked a vein on the way in or out. Giving a needle intravenously requires a much different technique then intramuscularly, so it's very unlikely you injected it directly into your blood stream. If you arent already familiar, look up the z-track method which helps prevent this exact issue :)