Sounds like what my son had, which turned out to be eczema. I would rule out that before changing your diet. My doctor suggested hydrocortisone, but it has a lot of nasty chemicals, so we only used it a few times (enough to get rid of it the first time). After that, we changed laundry detergent (I was using an all natural detergent, but I think it had something in there his skin didn't like, now use All Free and Clear), bathed my son less (maybe every 3 nights, it isn't like they get that dirty at 2 months, and we still wiped him down every night), changed his bath soap to an all natural one
(Nature's Baby Organics worked for us), and lotioned him up at night (again, go natural, you don't want the chems, we use Burt's Bees) whenever he was dry and had the eczema bumps. This fixed it for us, he did have a few "outbreaks" but we just used more lotion, sometimes hydrocortisone, and were really careful with baths. My son pretty much stopped having problems at a year old, but I know a lot of kids don't. I think we finally found a good balance to keep it under control. For us, it took maybe 3 or 4 days to get rid of it once we got rid of whatever it was that bothered his skin, but notice a slight change by the second day, so if you get rid of one thing at a time, you should notice pretty quickly that her skin is changing. Also, my son had problems where your daughter does, but also on his shoulders, upper arms, and back, mostly where his clothes rubbed a lot. You might want to try a looser or softer shirt, or no shirt/upper body cover, that might help too.