how do i no if my pup is growing to quick?

If I was you, I'd ask your breeder. They are normally the most experienced people. :)

There are a number of books on the market, sorry can't recommend any, that provide good information as to feeding large breed dogs, especially if you are not experienced with them (not suggesting you're not,but I don't know your knowledge :) ) It is vital that they don't grow too quickly as this is detrimental later in life.

HTH

she seems to have doubled in height in 6 wks but not a fatty yet dont want to drop a feed maybe i will move up to junior food for lower protein .And buy some more books.dont want her growing to fast because of the damage it can cause to her growing bones .

what breed is she?

she is a bullmastiff.