Pain on shoulder blades and below achy dull pain. can this be lung cancer or heart attack?how do i distinguish if serious or just strain?

Maybe gallbladder?: Depending on radiation of pain to and associated symptoms this could be due to gallbladder, stomach or heart. These internal organs have nerves which originate in the same place and therefore pain from one may mimic the other. Given your age I suspect its unlikely your heart and could be your gallbladder as that is a very common diagnosis. An inflamed GB can cause shoulder pain. See your doctor.

Here are few thought: Based on your age, neither heart attack nor lung cancer would be on my list of p. Most such aches and pains resolve in 1 to 2 weeks. All you need to do is monitor it &, take a couple of tylenols, (acetaminophen) if the pain is bothering you much. In case your symptoms get worse in 2-3 weeks from now, then you must go see your doctor who can examine you and do a chest Xray and guide you further.

Just strain: It unlikely that you have either lung cancer or heart attack. Are you a smoker? Can you do your daily activities? You can discuss it with your doctor next time you see her/him.