Unlike Trophies, which are permamently tied to your account, Medals are part of the weekly team competition. After a week ends, the top teams earn their prizes and Medal counts reset so that new teams have a chance to compete. Leaving a team will reset your Medal count as well.
Please reboot your device completely by holding down the power button for a few seconds and then sliding the slider that appears. If that does not fix the problem, shoot us a message using the form below and we will help you out.