Since every day life makes that our posture brings the pectoral in a position that are shorten/contracted, many of us have them tight. That can produce some pain in the back, between the two scapulas(shoulder blade) because the muscles between them and the spine( the rhomboids) are overstretch.Here are the different ways to stretch the pec:
-#1 :Stand in a door frame with one arm on each side of the frame. Do a step foward(with only one leg) until you feel a stretch(most people would feel that stretch where the arm is attached to the chest). Be sure to keep your back straight during the stretch. Hold that position for 30 seconds. Repeat it 3 to 4 times a day.
-#2: Stand beside a wall so that your body makes an angle of 90 with it(one of your side could touch the wall). Put your arm on the wall just behind yourself, just like the picture underneath shows, about shoulder high. You should be feeling a stretch, not pain, pain could cost an elongation. You can also do that movement a little higher than shoulder height and lower. Hold each position 30 seconds and repeat 3 to 4 times a day.
When doing ome stretch, you should feel it, but it shouldn't be painfull. Go slowly and respect your limits.