For The Last Four Days, When I Breathe In Deeply,

For the last four days, when I breathe in deeply, I can see a pulse/heartbeat just below my sternum. When resting I can't see it, but when working out it's even more visible. I'm very thin, but I've never been able to see it before. I've read a number of things saying this is common in thin people (I'm 35, 6ft, 124 pounds), but it's definitely new for me.

Possibly unrelated, I've had a "knot" in my stomach/abdomen, when I press below my sternum - lower than the pulsing there is a dull pain. It's worst in the mornings. It's not terrible, more just that I am "aware" of my abdomen during the day. Endo & Colonscopy were both clear. Trying a course of Prilosec to see if it's a weird acid-reflux issue. Lost 8 pounds or so since this started in May. Eating better, removed coffee to gain some weight.