Dozens != 20k. At a normal heart rate of 60 bpm, that's 1/4 beats in a day. And that's only an average. You might be getting strings of consecutive PVCs, which has some risk of sudden cardiac death.

One day, I got one out of four PVCs, regular as clockwork. That was... rather alarming. I knew what they were, but that was a ton more than I was used to. Yeah, I went to the ER for that...

What was the end result? Did they go away?

They went away that day after a few hours. They went away in general when I quit taking Claritin.