 [ Symptoms ] i have a small lump on the back of my thigh its dark blue and it hurts i notice a small lump on my thigh its dark blue should i go get it check or just watch it closely [ Disease ] valtrex will not help a rash related to mononucleosis there are thousands of medical conditions that can result in a rash so it would not be wise to associate your son is mono with your rash the rash seen in mono does not usually itch since i cannot see your rash and examine you over the internet i would advise you to see a medical provider so you can be properly diagnosed