It could be, you can always give it a try and see how she does with it. The age guides on the packets are only rough guides, some babies stay on slow flow, some need faster earlier.
If you think it's the flow of the teat try the next size and see. She may need to have a few feeds with the new teat to let her get used to it before you decide if it's suiting her or not.
Let us know what you do and if it helps!
Laura