A few wild guesses.....the tub/shower drain could be lacking a trap in the drain, but if this was the case, you should have been smelling sewage right from the beginning. Another possibility is that something is still plugged and sewage is backing up into the shower trap even though it doesn't come up where you can see it. Another is that the shower isn't properly vented and the water is getting sucked out of the trap.
The purpose of the trap is to allow the water in the trap to act as a seal so that sewer ordors cannot come into the living areas of the home. So either there is sewage backing up into the trap or for some reason there is no water in the trap. This assumes that you have cleaned the shower floor very well so there is no sewage smell there.

hi coolvt, our contractor said that all of our drains have ptraps. so far, the shower and all of our other sinks, etc, drain quickly. we had our contractor come back and he cleaned out the shower drain and flushed it out with water as well. we checked all of the cleanouts around our house and they seem ok, so i don't think there is still a clog anywhere anymore. i also scrubbed the floor and everything with bleach and soap and doused it with boiling water, so i don't think the smell is from residual sewage. but in the hot afternoon, the entire bathroom still fills up with an overpowering sewer smell or gas. when it is cooler outside, the smell lessens to some degree. do you think it is from a leak in the ptrap or a venting problem? how do we figure out which it is? thanks so much for your help!

Check underneath the house to see if you have a leak, and check the vent lines to see if they are clogged. A clogged vent line will result in the water being sucked out of the P-trap for that fixture, allowing sewer gases to come up.

The fact that it is worse on a hot day indicates to me you have a leak, but if you had such problems with clogs throughout the system, it seems logical that vents could be clogged too. Perhaps a family of rodents moved into the pipes during the renovation.

remodeler: I think he said it was a slab house. Possibly the pipe is cracked under the slab. Try this: In the afternoon, when the smell inside is worst, walk around the perimeter of the house outside and see if you smell anything near ground level. If you do, that indicates that sewage is coming out of a broken pipe and leaking underneath the slab. You'd have to get the plumber in with a camera to find it.