I can't take it anymore!

My LO is 4mo and 2 wks. She has been biting me every time I nurse her for the past 2 weeks. She'll suck a few times then bite me like a chew toy. It hurts so bad! I've tried everything. Tried saying No! With a stern voice. I put her down when she bit and tried again. She just keeps doing it. When I yell No! She just looks at me and smiles. I don't wanna stop nursing her but I can't deal with the chewing. The last two weeks I've been exclusively pumping and I hate it!!! I've already gotten 4 plugged and painful ducts that keep reappearing. Help! I feel like a failure:(