Started taking Focalin XR 20mg yesterday. I noticed yesterday and today a shortness of breathe, I think, because every once in a while I have to take a really big breathe. Do these stimulants really slow down your breathing rate and cause this? (Checked the blood pressure it is still right on 121/82.)

This ('dyspnea'; or fast breathing too, called'tachypnea') can be a stimulant side effect. (I had it on Ritalin.)

Talk to your Doctor.

It's a normal side effect. Tight chest/taking deep breaths/feeling 'tense' (you could call it tachypnea, but it's not exactly that)