New diet, and exercise can make you very healthy...
But what does it do? IT CLEANS TOXINS that were in your system. Basically what is happening is you answered your own question.
Pimples,
redness, ichyness is caused from "toxic" sweat. (Toxic as in, not healthy as it is getting)
You should wash your face LIGHTLY (remember, SCRUBBING SPREADS PIMPLES!) Wash your face with anti-bacterial soap of any kind (as long as it is not the scented expensive kind...The cheaper, the better...REALLY!)
You can also try
Benzoyl peroxide cremes (I reccomend the lowest... which is 2.5%) since it is light.
ONLY PUT IT ON AFTER A CLEAN SHOWER AND SHAVE, OR at least shower and a nice face wash. Just be careful, it can "bleach" your clothes if you put too much on.
Good luck! GET IN SHAPE! - Also, try eating more
Vitamin C, and maybe some Apples and Bananas daily. That can help a lot!