This is a very common complaint. we spend so much time in the arms-forward position between the computer and driving. the chest muscles become tight and constantly pull on the oppossing muscles. that constant day after day pulling up front forces those back muscles to react and they react by tightening up into knots. massage on the painful part is helpful in the short term but for long term relief you need to stretch that chest out.
a great tool for that is called a foam roller. it's a therapy tool you can buy at walmart or online. it's a 6 inch diameter foam noodle. you lay on it. place it on the floor and lay you spine and neck and head on it and let your arms fall to your sides outstretched. you will feel it in your armpit area and the sides of your chest all the way to your sternum. stretch for as long as you are comfortable with. many people get instant relief from back pain with this kind of stretch.
another great tool for a similar effect is the swiss ball. stretch out backwards over it and outstretch your arms like on the foam roller and relax for as long as you're comfortable.
hope it helps!