baby troubles

This is what Ana's back looked like yesterday.
Today, it was mostly on her arms and legs. I noticed a slight rash about a week and a half ago, and when we went to the doctor last week he gave me a prescription cream for it. But this week its gotten a lot worse. I talked to my sister (who is a nurse) and her husband (who is a doctor and has specialty in skin) and they told me it looks like a systemic allergy. In other words. She's allergic to something she's been eating. Its either eggs, since they are in everything and she's eaten a lot of stuff with them in lately or peanut butter which she's had before, but more often recently). So, I have to keep her away from those things. If its not gone in a week then it might be milk, but my sister didn't think so. On top of that, she's been seriously constipated for a while and today it was so bad we kept her home from church. So, here's my dilemma - my daughter is underweight and I need to fatten her up, but I can't give her peanut butter or eggs due to allergies or cheese, bananas or applesauce since those will worsen her constipation. So, I'm stuck. I feel so bad for her, I just want her to be healthy and happy.