It should be around 16cm from the pubic bone, but anywhere whithin a few cm is fine.
I only just started to feel mine about 12 weeks too. I'd wait til your next appointment before you even think about worrying.
I'm sure when your doc measures it'll be right where its supposed to be, or within normal measurements
Oh & its from the middle of your pubic bone too, the top could vary, lol. As long as you can feel it I'm sure its right where it should be