[wingnut60]

No water under the toilet at any other times? There should be an access panel some where on the backside of the shower to get to the faucet connections--could be a leak there. Since it is only after using the shower, and the shower lines have pressure on them up to the faucet, I would think you have a leak in the drain. There should also be some access somewhere to the base of the shower to where you can see if the drain fitting has loosened or is cracked.
This is all assuming that the water is traveling under the linoleum from the shower to the toilet area.
Need to do a little searching for access to the bottom/underneath of the shower.
Joe