My dd did this, I got mastitis around the same time due to getting blocked ducts. She just wasn't feeding effectively. I put it down to teething, and it sorted itself out. A week or two later the behaviour stopped and it was pain free again.
Dd is 20 weeks (time has flown!), and in the last few days has been a nightmare to nurse.
She has developed a bad habit of pushing on my breast with her top hand, whilst arching her head away for maximum pain for me!
The milk is there. I have tried hand expressing so she's not getting impatient for a let down. She generally does it after let down, though, once it's getting more to a comfort feed rather than actively feeding.
I have just replaced boob with dummy as she was doing it a lot there, and she's fallen asleep almost instantly.