A proper fix would have been to have a professional plumber come out and do the work, As one of these I would have to say nothing is "beautiful " about a system that was imoproperly designed, regardless of the cosmetics of what the "plumber" you had did.
Okay, the issue.
unvented or improper venting causes a burping effect in the drains ( think 2 liter bottle turned upside down ) so the heavy materials ( like #2

) remain behind in many instances and, over time cause enough buildup to restrict flow... this causes backups like what you are experiencing.
With a Proper Vent installed the waters flow is Considered "laminar" ( meaning smooth ) as compared to prior to the Vent installation which would be turbulent. and so material can run smoothly through.
What to do now,
Start by having your drains snaked as they are fiull of crap and this will not go away without proper cleaning, regardless of vent installation.
if you Can, have a proper vent installed... If this is not possiblle Then try puting and Air admitance valve ( commonly referred to as a "cheater vent" _ on the place where your bathroom sink comes up, it should be a T with the vent coming off the top as high up as practical and downstream of the p trap for that sink.
If your plumber missed installing vents and the importance of these vents then I wager he Also didnt Run the DWV properly and simply adding a vent may not be enough to prevent Re occurance.
1. Call plumber to snake drain.
2. get them to assess the drainage problem.
3. stop having poop in your shower...
If you really want to SEE what is happening in your drainage Take a 2 liter bottle and fill with water, tip in upside down... This is the system without venting. now poke a pin hole into the bottom of the bottle and invert it again. this si the effect of adding a vent, it allows the atmospheric pressure to remain the same on the traps and drainage piping. \
..Sorry for the long Writeup, I got a busted leg right now and am Bored...
Hope you enjoyed Plumbing 101