Since you last paid for your retainers, did you add another platform to your account? (PS4, PC, etc).

The reason I ask is because this happened to me a while back and it was because adding PS3 to my account added 30 free days to my account, but it then meant my subscription was no longer lined up with my retainers (and it wants to charge for retainers at the point it charges for your subscription).

Heading to the mogstation I had to do a manual small payment for the retainers to bring them back in line and since then I haven't had any problems.