Sprint Navigation free a few days a month?!

[DEFAULTx]

Don't know if anyone else has experienced this but ever since I purchased my phone on November 1st it seems that Sprint Navigation works for a few days at the beginning of the month for free then all of a sudden starts prompting me to buy. I have an unlimited data plan not a SERO plan. I know there's been discussions about whether the service is free for SERO plans but it just seems odd to me that it'd work for a few days & then stop. So far I've noticed that it works at the beginning of the month then stops. I'm keeping tabs on it this month & it's on it's 5th day. I really like the app although having a VGA version would be better but I also use Live Search & Google Maps. I wonder if there's something that changes on my phone to make it work for free then stop.