My 16 week old pup can only hold her bladder for 2 hours in the crate. She has been checked by a vet and there is no urinary infection. What can we do?

I read all the info on your site, but didn't see anything in regards to my particular problem.

I have a 16 week old Australian Shepherd, Lab mix who is completely house trained and crate trained. She now rings a bell on the door when she needs to go out. However, she needs to pee very frequently - about every 2 hours or so (but can hold it all night). So, when I have to leave her in the crate for over 2 hours, at a certain point she cries and cries. I believe because she simply cannot hold it any longer, and is forced to pee in the crate. Under two hours she is fine - no noise, no peeing. The vet has checked her for urinary infections, etc. and all is well. I have tried the crate with no bedding, but after 2 hours she will still pee, and in this scenario gets stressed out because she doesn't like to sit in it. Occasionally, I have meetings and need to be gone longer than 2 hours. What am I to do? Will she grow out of this and be able to hold it for longer periods? Should I leave the bedding in or out? Help!