What Causes Pain Near Lower Side Of Pectoral Muscle?

Every now and then I get a little pain near my lower side of my pectoral muscle. I thought that it was something to do with my heart but they ran multiple tests and ruled that out. Said I have a very strong heart. Said that I should keep an eye on it but did not tell me what they thought that it could be. I am obese and am working on getting healthier but do I need to do something else. What could this be?