Possibly a clogged shower head….remove showerhead, clean out debris, soak it in vinegar overnight, reinstall.
Maybe debris in the valve body at the cartridge(s) or stems/seats. Remove cartridge(s) or stems/seats, have someone slowly open main water to flush valve, reinstall new cartridge(s) or stems/seats. Note: I recommend new parts since you’ve got the old ones already out.
Could be a clog in your waterlines. This usually occurs with old galvanized waterlines. Get a plumber….
There are other possibilities. No way to really determine what the problem is without seeing it. If you dont feel comfortable getting elbows deep in this project, you should call a plumber and let a pro remedy the situation.