I believe you, I remember this little problem from ExoTeric. For starters, let's assume your X is the greatest thing since Kevin Mitnick.
- Did you scan your box for malware? If you haven't, do so now.
- Are you on a network? Could she be running a packet sniffer? If so, get off the network.
- Are you using a pass easy to guess? If so, change the pass.
- Did she steal your cookies/hijack your login session? If so, delete any and all cookies related to T-Mobile
- Could she have gotten your pass off your phone? I'm not too sure if that's possible, but you gotta consider everything.
- Could she have used a known password to the email/whatever you use when you want to reset your T-Mobile account password? If so, change that pass. And preferably use a new email/whatever to reset your T-Mobile account pass.
That's all I can think of. Hope you get it sorted out.
"Workers of the world unite; you have nothing to lose but your chains." -Karl Marx