Why did I get an intramuscular haematoma after a Diclofenac injection?

When you had your injection, the needle peobably went trough an blood vessels. When the needle was withdrawn there was bleeding from the vessel puncure. The blood accumulated in the muscle. This is an intra nustcular haematoma. Drugs such as diclofenac can also lead to thinning of blood increasing chances of bleeding.