I had my back tyre replaced yesterday because it had a hole in it and was causing the tube to blow out. Now when I ride it, every time the wheel goes around, there a particular spot where it feels like i'm going over a dip. So when I'm riding fast, it's all bumpy and stuff. I've checked the spot and there doesn't seem to be anything obviously different about it. Any ideas on what it could possibly be? Do I need a new rim?