What can cause a pulse rate of 48?

It depends: This could be a "normal" rhythm just going slow - sinus bradycardia. This may be due to medications, thyroid issues, high exercise conditioning, or from a "sick" sinus node. The pulse may also be slow if there is slow or absent communication from the atria - partial or complete heart block. This could be potentially quite dangerous. An ekg should be able to tell for sure.

Few things: Athlets with exellent exercise tolerance have low heart beats which is normal. Pathologically low heart beat is seen with patients with nodal system dysfunction. Medication such as betablockers can reduce heart beat as well.