Intramuscular injections are known to be sources of intramuscular infection, cellulitis and abscess The early signs will be constant throbbing pain, increased local temperature, visible swelling, redness etc. As i understand, U have none of these. Hardness or induration is the normal reaction of our tissues to any injection.. The more the reaction, more the hardness So don't worry hardness will stay for some time and wean by its own.

Your symptoms are suggestive of your condition forming a likely thrombus(blood clot). You can apply ice over the area for 10-15 mins 3-4 times a day which will help reduce the inflammation, also apply a hot water bag over the area after 30 mins for 10-15 mins which should help reveal the pain and hardness. If there is a development of heat , redness and other such developments over the area visit the ER at the earliest.