Is my 11 month old weaning? Please help!

My baby girl was exclusively breastfed for a month. She was gaining very slow so I started her on formula, as well as letting her nurse whenever she wants. She's always loved to nurse but around 8 months, she mostly only wants to nurse to sleep. She is now 11 months old and when I try nursing her for her naps, she keeps smiling at me and biting my nipple, but when I put the boob away she cries like she wants it. But then she bites me again and again. So just now I rocked her to sleep without a boob. Does it seem like she's weaning? I want her to nurse as long as she wants but she keeps biting! Here's my little biter.