The med they gave me is rythmol. It is supposed to help regulate your HR. It has very mild beta blocking tendencies.
There's your answer- beta blockers slow the HR, and if you're on one, you'll see that your maxHR (not calculated, but actual) is considerably lower than without a BB. I was on one about 10 years ago for ventricular extrasystoles, and could never get my HR above 140, no matter how hard I tried! Went off the BB about two years later, and within a month was getting my max up in the 180s. Yeah, yeah, I know- at 48 (my age then), I (theoretically) wasn't supposed to be in that range, but I tend to agree with Gabe Mirkin on this- if you want intervals to do their job (and you don't have access to a cardiopulmonary lab to measure your VO2 while you work out), you have to be close to that range during a bout.
Anyway, nothing to worry about, although my cardiologist friends/colleagues don't have an answer for the questions of whether or not your myocardium is really benefitting from strenuous cardio when you're on a BB, becuase your HR isn't near your estimated max for your age. I agree with what Velo Dog's doc said!
Oh- BTW... afib is associated with irregular systoles, so some HRMs could (at least theoretically) underestimate your actual HR, which calculate the HR from the R-R interval (that's the distance between the "big" waves on the EKG). Most HRMs average R-Rs though, so it's probably not a problem.