Rash or Milk Sensitivity...?!? Mother of 3 feeling suddenly like a newbie!!!

Baby Maggie came home with a little bit of baby acne, but nothing major. Over the last 2 weeks she's been struggling with gas, and the need to burp immediately after a feeding due to spitting up a lot from my overactive let down...BUT now we have been worried that she is developing a rash, she's had a bath with baby soap twice during the week (tues/sat) problem is it seems to have gotten worse and spread to her arms and chest. My older sister mentioned a milk sensitivity like her son had which took her 6 months to figure out years ago, but which she solved by eliminating milk or dairy from her diet. Any opinions would be great, I'm a 3x mom, but I feel like a newbie on this one and it's already been a stressful week. older daughter has been sick and son is always a handful, I'm getting no sleep and hubby is working full time through massive sleep deprivation. I really don't wanna have to rush her to the Drs again (we've been in three times in the last two weeks for one or the other if the kids- I don't wanna be a crazy mom who is there every day, but I'm getting worried! Here are some pics to compare: