Some individuals do in fact bruise easily, they may have an ascorbic acid(Vitamin C) deficiency or some coagulopathy. Perhaps they are taking a blood thinner medication like Coumadin, ASA, or Lovenox.
Anytime the tissues are traumatized bruising is possible. Receiving a shot IM or SQ is trauma, albeit very little trauma, nevertheless still trauma. Unfortunately though, a common cause of bruising after receiving an injection is poor technique by the person that administered the shot. Blood slowly leaks into the subcutaneous, dermal and epidermal tissues and causes a bruise.
A bruise is basically a very minor injury that results from leaking blood vessels just under the skin. This blood leaks into the surrounding tissue and forms a bruise.
Many times when people have blood samples taken, they develop what is called a hematoma. A hematoma is when the blood leaks a bit from the vein that the blood was taken from. It forms a lump under the skin and eventually looks like a bruise.
Usually, bruises do not need any treatment but if it is painful ice can be placed on it to help alleviate any pain.