There is not timer so I would think that one of two things is happening...
1. You are accidently turning off the water to the shower head at the head. On the back of the shower head, you can turn off the water by turning the flat disk shaped thingie.... or
2. The faucet plug is not staying up so the water reverts back to the sink fixture.
Have you looked at both of those?
Chris, I would think I am turning off the shower head at the head. I did not know there was a turn off there. When done showering I place my towel over the shower head to catch any dripping. I think I might bump the shower head enough to turn it back on for the next shower. I will be checking that, and thanks so much for the answer. I am not very observant I guess.
Hopefully I will have a nice longer shower from now on.