The most common causes of first degree heart block are an AV nodal disease, enhanced vagal tone (for example in athletes), heart muscle inflammation (myocarditis), heart attack, electrolyte disturbances and medication. The drugs that most commonly cause first degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include calcium channel blockers, beta-blockers, digitalis cardiac glycosides and anything that increases cholinergic activity such as cholinesterase inhibitors (pesticides).
Do you have some of the things mentioned above?