need some advice asap!

My little girl is about to be 6 months, but according to the pediatrician she's developing like an 8 month old! She can sit up and roll over and is already eating baby food. I'm pretty sure she's got allergies and nothing seems to be helping her. I was wanting to try a small dose of the children's Benadryl. She's teething too so I thought t might help her fall asleep because she's been having trouble doing that too. Anyone else use Benadryl on a baby? Maybe like a small dose?