I know you weren't asking me Hazelnut, but I figure this is my area of 'expertise'. My resting heart rate is between 50-65bpm.
When I get nervous, or start to feel my heart pound hard (not necessarily fast) that in itself makes me nervous. If it gets over 100bpm I know that I'm on the verge of losing control (assuming I'm not doing something physically demanding, and just sitting there)...normally I feel the surge of adrenaline course through my veins and I get to around 135bpm and then I can really feel myself going into that 'panic' place.
I've gotten it up to 200bpm and that's as high as my heart rate monitor watch would go...and I started to black out at that point. If it goes over 170bpm I normally need ambulatory assistance, although thank goodness that's only happened 4 times.
So for me it's a combination of speed and hardness of the beat. Last night I couldn't sleep because it was pounding so loudly that I could feel it no matter if I switched from laying on my stomach to my side to my back. But the actual bpm was only between 70-85bpm. This is considered normal of course, but high for me when laying in bed.
my heart has pounded over 150bpm without doing exercise for over 4 hours before. The entire time I thought I'd have a heart attack, but one never materialized. i've had a heart holter test before and everything was fine..blah blah blah...
I think everyone can react differently to all drugs, so the Guaifenesin may have raised your heart rate slightly and then your mind did the rest.
I also get a much faster heart beat when I'm sick. My average goes from 50-65 to around 75-95bpm. So to me that's normal too.
Take care :)