I'd be calling my doctor if I were you. It is possible to have a heart attack
, even at 18 (My first heart attack was before I even began elementary school, so it is possible.)
You might want to consider taking aspirin
or something, however, call your doctor first and see what he/she thinks before you take ANYTHING.
I really don't think the antibiotics would give you chest pains, I've never ever had that experience with them and I've take a lot of antibiotics.
It may just be the heart murmur
, but you won't know until you see your doctor.
Best of luck :)