What Causes Burning Sensation On The Skin While Taking Heart Medication?

My husband at night in bed often feels like his skin is on fire . He doesn t feel hot to me. He had triple bypass over 4 years ago and is on the typical medications that go with that condition. He has allergies. This has been going on for over a year now. We bought an old-fashioned mattress, no foam, but that hasn t helped. Some nights he is fine. Most nights are a real struggle.