cardiac anxiety (forget to mention this in last post)

17M, I randomly get this weird pain feeling (doesnt exactly hurt) but like a sudden hard feeling in my chest/heart area and it feels like everythings gone dark and im about to fall and pass out, i feel like im going into a cardiac arrest or a heart attack but then it fades away with my heart beating super fast, and it happens when i stand up sometimes, while walking and even while sitting, note: (i have bad acid reflux and gerd atm and am on meds for it)