Were you using an inhaler? The steroids can cause this mouth problem and you are supposed to rinse your mouth out after using steroid inhalers. Steroids can definitly cause rapid heart beat. Still, I would call doc. about sores. The sores sound like they could be shingles.
If your mouth is tender inside, you may have thrush due to the antibiotics. You can try eating active culture plan yogurt, or call your Dr. for an Rx for Diflucan. As for the heartbeat, have you been taking prednisone? This can cause rapid or uneven feeling heartbeats.