Couple days after getting my blood drawn from my left hand, i've been feeling pain in that arm especially when i stretch it out. what's wrong?

Depends: If there is discoloration, swelling, numbness or marked tenderness of the extremity and along a vein, the arm should be examined. Things like infection or clots would have to be ruled out by a physician. If there are none of these findings probably nothing serious other than some positional muscular strain is present and the discomfort will clear on its own.