Why won't my toilet flush right? Did our plumbers not put something back right?
If you had "the sewage line get backed up" all along and didn't do anything to fix it (replacing the water line doesn't) then the sewage line will only get worse. 

As for your tub (I think you want me to guess you shower in the tub instead of having both a tub and a separate shower), it probably connects to the sewage line after the clog that affects your toilet.