I bought a new hybrid bike (Saracen urban X2) and after two days, the rim of rear wheels got trouble. The shape of the wheel slightly changed from circle to earth like shape. A very little but I can see the bump. I ride with the bike only on road not that bad condition. (Some part of the road is not very smooth but cannot say rough road). Now my question is how this can happen? Is my weight (220 pounds) too heavy for riding this bike? I am not an expert about bicycles. Please help.