I believ they ae also known as PAC or Premature Atrial Contraction. It's a premature heart beat that initiates in the upper chambers. I've had some of these, and it feels like a skipped beat similar to a PVC, only the "bump" you feel isn't as pronounced.
Supraventricular premature beats or pac (premature atrial contractions). Your slow heart rate is giving another part of your atria that is irritated an opportunity to fire off another errant beat. They are of no consequence in an otherwise structurally normal heart. Do you get short of breath or have feelings of dizziness or faintness?