Hi, a few thoughts.
First off, 6 weeks is growth spurt territory so if you're seeing an increase in feeds it could be that.
Are you burping her well after the df? I know that Tracy says in the book that LOs don't always need to be burped after, but I found that mine did -- also you might look to use a different flow bottle teat if you're not already using the slowest one.
Changing the time of the df makes a difference for some -- 20 minutes earlier in your case might make a difference.
And some babies just don't do well with the df, so in some cases it is better to just stop it and go without.