I'm on the $30 plan, the OP is on the 1GB simple choice plan.
Everything appears to be back to normal for me today though, with Google Play Music no longer racking up data in the T-Mobile app. I rebooted my phone last night, but I have no idea of that has anything to do with it. Have you tried reinstalling the app just to see if anything changes? Not sure what else to suggest besides trying to contact T-Mobile again. Sometimes if you hit them up on Twitter (as well as CEO @johnlegere) they'll jump a little higher.
By the way, it shouldn't matter if you're streaming music that you uploaded to Google's servers or not - none of it should count towards data usage with T-Mobile as long as it's coming from the Google Play Music app.