I already had an XBox live account on my old Xbox. I converted it to a Xbox Gold account.
I figured that it would just turn my account into a 360 gold account. Low and behold, 2 months later I get a 49.99 charge on my credit card for Xbox Gold!
So, having an existing account and transferring it onto the 360 still makes you pay a $49.99 base fee, plus monthly charges?
When did your subscription expire? That might be the reason why.