1994 grand am-exaust help needed

I recently worked on my mothers exaust and noticed that all bolts that hold the I believe catilatic converter together with the resonator(or hollow round piece) behind the converter toward the rear of the car were rusted out.so I cut that round piece out and put in a straight piece of exaust in its place,and now it is running really bad. Would that round piece be the problem and should i have replaced it rather than bypassed it??? Please help she is very mad at me now.

That shouldn't have any bearing on what is happening. Did the exhaust leak in that area when you worked on it? If the answer is yes then I suspect that the cat is probably blocked and needs replaced. Prior to work the exhaust was probably excaping through the opening in the exhaust system. After it is forced through the cat and if it is blocked the performance is suffering. If it didn't leak then check to make sure that you don't have a collapsed muffler or blocked pipe.