L.L.
Just because there is no history in the family (and you don't know that...it's just that probably no one has been tested!) do not rule out Celiac disease. My two year old has Celiac disease, and we don't have any "history" in the family...but a long history of Crohn's disease, cystic fibrosis, and other conditions.
Symptoms for my child while consuming gluten (and they showed up right away when I switched from rice to barley infant cereal) were failure to gain weight, constant explosive diarrhea, skinny legs, arms, and butt, bloated tummy, almost constant pain (evidenced by hours and hours of screaming, thrashing, inconsolable fits) and the most awful dermatitis you've ever seen...look up "dermatitis herpatiformis." Literally, it looked like her skin was rotting away in some places.
After putting her on a strict gluten free diet, her symptoms improved almost immediately. It was like a miracle change. This unhealthy, unhappy baby began to thrive...to walk, talk, grow, smile, laugh, and play...things she hadn't been doing. We had to put her back on all the foods for the testing, which broke my heart, because he symptoms immediately return. She is so sensitive that even a touch of something processed on the same line as a gluten bearing product can cause a full blown attack in her.
Testing at a young age can be unreliable at best and they need to be consuming large amounts of gluten for at LEAST several months prior to the test. If you don't want to go through those things and you see such a marked improvement, just make that leap to a fully gluten free/dairy free diet for your child. My child is also moderately lactose intolerant as a result of the Celiac disease...when she is fully healed she will probably be able to consume dairy products but for right now, we are doing lactose free for her.
If you would like any help with the gluten free diet, please PM me and I'll offer you my best advice and tips, as well as foods we have really found to like and meals for the whole family that are gluten free.
I know how awful this can be, I do understand, and I'm here if you want to talk. :)