If you came off over poles, then more than likely your seat is not secure, and your balance is lacking.
Trainer is obviously seeing something that if not going well in your riding, unsteady hands, jiggly seat, riding off your hands, but whatever it is, they are doing you a huge favor by not letting you get ahead of yourself.
What everyone else said- speak to your instructor. I started cantering after about a month or so of riding :) It really varies from person to person, as well as instructor to instructor. You may think you're ready, but are you really?