What Causes Red Bumps On Stomach And Back?

I have had lots of red bumps on my stomach and a few on my lower back since Tuesday night (5 days ago). Other than a little burning sensation, they haven't bothered me too much, though it feels and looks worse at night. During this time I have felt a little achey with slight headache with intermitant sore throats that wake me during the night. I have taken Benadryl, which may be drying out my throat.... Tuesday I hiked up a snow laden mountain and became very hot, though the bumps don't have that red rash look that I've had with heat rash... Wondering if I should see a doctor?