there is a possibility that there may have been multiple attempts at the same site before taking the blood sample. this causes extravasation of blood in the surrounding tissues as well leading to hematoma formation often accompanied with discoloration of skin. this may also cause increase in compartment pressure (caused when pressure builds up within the muscle) cause pain especially while stretching. you may use hot fomentation locally over the site for 15 minutes each thrice a day. definitely you will be benefitted from this. for further doubts consult a family physician online --> <link>
what is the reason for my arm pain?