Fever and a rash-is it serious?

My son recently cut a tooth and subsequently has been running a fever. Today I noticed he has a slight rash on his stomach and groin area and also a few spots on his arms. His temp is about 101 and hes acting like he doesnt feel well, whining and clinging to me but he also has spurts where he plays and laughs and it seems hes feeling fine. i also noticed that he has a a little diarhea too. I am making him an appt. on monday, but in the meantime i was wondering does anyone know if this is serious or what it may be??