We've had issues in the past about having the sewage line get backed up. We just had the plumber here almost a week ago to replace the water line. Now our toilet won't flush right. It flushes all the water into the bowl but it just stays there and slowly goes out of the bowl but it still leaves solid waste behind. We have tried our normal "fixes" when the line gets back up but it's still not doing right. We plug the shower when we take one so the water doesn't drain and then wait overnight to flush but it does the same but then we unplug the tub and the water there drains perfectly fine. What I'm wondering is if the plumbers did something under the house that affects the pressure of the toilet flushing.
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.