Hello~The muscle pain you are having could very well be due to running again after not doing it for a long time. You could have some stiff joints and muscles and running is causing the pain to act up. The heart would naturally pound as your body is not use that type of exercise anymore. The heart would beat faster anyway as the body is using up oxygen.
The fact that your pains last only seconds is reassuring, normally, heart attack pains last and get progressively worse. The pain could also be due to being uptight and worried over having the pain, thus, causing a chain-reaction so-to-speak.
You say the pain can also be caused when you bend down or lean over, well, that could very well be due to a hiatal hernia; a hiatal hernia can give you pain that mimics that of a heart attack.
I don't think you have anything to worry about, but if the pain keeps up, I would ask your doctor and explain about the pain being there when bending down, he/she possibly will test you for GERD or a hiatal hernia. I doubt if it is heart-related.
I hope you feel better soon. God bless you.
Going through the same problems as we speak...sucks. I am waiting for a call to get a cardiac catheterization to check since my nuclear stress test came back abnormal. Cardiologist didn’t seem concern but we’ll see what he says.