Never satisfied

Alexandra
My daughter is 7 weeks old I pump and bottle feed her but she strictly drinks breast milk. The past 2 weeks she just never seems satisfied or full she will eat up to 9 oz in an hour and a half span and still want more to the point where i have to stop her because she will just keep going until she spits it back up. But is still fussy in between. 
At night she will drink 4 oz and sleep for maybe 2 hours then want more. I'm exhausted and can barely keep up with her demand. Is it too early to start mixing in rice cereal with her milk or should I suppiment formula every once in a while? Pumping and bottle feeding is demanding and stressful as it is but with how much she wants its starting to seem impossible.