It is possible that the cat was simply burning the backed up junk that had accumulated in the engine. I would pull the spark plugs to see what they look like and replace as necessary. Usually a glowing cat indicates that there is too much fuel in the exhaust system. This can be caused by a sticking injector or bad plugs that are not burning the fuel mixture well. The new cat is probably fine. Check the muffler and tailpipe to make sure it isn't blocked.