hey there Topiczj, I had the same issue, but without the awesome military discount plan. Thank you for you service btw. If you changed the plan before you paid your account then there is the issue. So you pretty much got charged for last month and this month since it is prepaid. I don't get that either but isn't both plans prepaid since there is no contract? it's like a new bike that is rented, but in order to ride. There needs to be paid first before you get to ride the new shiny bike. The old bike either didn't get paid yet and they went ahead and charge you for that month on top of your new bike. I know my new shiny bike had a double charge and i was mad. but after figuring out that since i used the line before the bill date for the other bikes. I had to pay them for usaged that month on top of my single bike.
But try twitter or facebook for better answers. I know i Did. I had a awesome magenta dude explain step by step on why my bill was so high. just link them this thread since you have already stated your situation!
p.s it wouldn't let me edit my post...... grrrrr