There are a few reasons why this may happen. The most common reason is that the orthodontic appliances orbracesare rubbing against the lips or the inside of the cheeks. There could be another reason, such as an allergic reaction to nickel. This is very rare but may happen. If you are allergic to nickel, you are not a good candidate formetal bracesbut can wearceramic, nickel-free braces, orInvisalign. In the case of braces rubbing against the lips or cheek, speak to your orthodontist. He or she can easily adjust the appliance to stop this from happening.