Colic? Cramps? Allergy? SO IRRITABLE! :(

So my son is barely one month old, but he's not sleeping more than 1/2 hour at a time. He also has very sudden screams that resemble gas pains, sometimes in short bursts, sometimes lasting for several minutes. This just started within the past 4-5 days. Before that he would sleep for hours on end, and a touch of gripe water cured all cramps. We thought perhaps it was an allergic reaction to the formula, so he switched to a soy based mix this morning and so far no improvement. He is not having bowel movements often enough - perhaps once every 24 hours or so and he has a rash on his torso that seems to resemble heat rash. He hasn't been in the heat lately at ALL. I should add that he is breast/bottle fed, more intentionally bottles in the evening, however it's increased more lately with not enough breast milk to satisfy his hunger. We took him to the hospital tonight where a doctor told us it's heatrash. Period. They gave him a suppository and sent him home. Baby put a little swipe in his diaper when he received the suppository, then there's been nothing since. It's almost as if he's hurting from lack of bowel movements.

Does anyone have any suggestons as to what could be making him so irritable or how to fix it? Thank you for any answers in advance.