The rash could be any number of things ranging from allergy to gvhd, best to have it looked at by the doctor quickly as skin gvhd can spread and worsen rapidly.
GVHD is very common with about 35-50% of transplant patients developing acute gvhd ( gvhd within 90 days of transplant ) and cGVHD (chronic - occurring after 90 days) about 30-50% of transplant patients. Skin GVHD is the most common with about 80% of GVHD cases involving at least the skin, about 50% going to the gut and 50% to the liver as well.
GVHD is graded in severity from grade I to IV, with IV being the worst scenario. Skin GVHD is graded based on how much of the body it is covering and how severe the symptoms are - ie, how red is the skin, is it getting tough and itchy, is it developing blistering, etc.
Main point of it all is ask the doctor asap about it and have a full list of symptoms = when did it start, is it itchy, painful, is the skin tight, did it start in one small spot and grow, or did he wake up one morning with it all over the current area? Also, is there anything that aggravates it (hot water, cold water) and anything that soothes it?
Hopefully you'll get this sorted out quickly. I have had some form of skin and gut GVHD since transplant over 5 years ago with a few short-term breaks, but I fit chronic GVHD pretty classically.
MDS RCMD w/grade 2-3 fibrosis. Allo-MUD Feb 26, 2014. Relapsed August 2014. Free and clear of MDS since November 2014 after treatment with Vidaza and Rituxan. Experiencing autoimmune attack on CNS thought to be GVHD, some gut, skin and ocular cGVHD. Neuropathy over 80% of body.