Then I shall give you the reassurance you are seeking, as will everyone else here!
You had coffee. (A delicious substance many of us need but is well know for it's ability to upset the digestive track and to contribute to anxiety) You got gas. You burped, You felt better. You were "rushing around." You felt out of breath. You stopped and took some very deep breaths. This was good, but you MAY have taken too many and did, indeed, "overbreath" causing an upset in the balance of oxygen and carbon dioxide. This can lead to anxiety, (helped along by that big "cup of joe") which can lead to feelings of chest discomfort, which can lead to heart palps, which can stress you out, which leads to more anxiety...........
Your heart is fine as the cardiologist told you. I believe your anxiety got the better of you and perhaps you should try cutting out the coffee and see if things don't improve. But if you are a daily drinker, (of COFFEE) quitting cold turkey can cause some nasty headaches, so perhaps you could just try cutting down on the amount for now. Even if you only have one cup in the morning, but have that one cup every day, then you are one of the dreaded "java junkies" we all read about. Tomorrow, only fill your cup 3/4 full, see what happens. Decrease the amount slowly and begin to add decaf to the mix until you are just on decalf. It's also very possible that this was a one time "thing" and has nothing to do with coffee. You could very easily just have had a minor anxiety attack. I would, at this point, try really hard to put it out of your head and see what tomorrow brings. If this keeps happening, I'd blame the coffee since we know your heart is sound. Maybe check out the stress level in your life. Could be over the top right now.
I really think you're fine but would urge you to NOT begin to obsess about this incident.............those of us with anxiety who foolishly go out and look for it, usually find it.
Peace
Greenlydia