Surprisingly I was billed for £94.75

I have been a customer of T-Mobile coverage for 18months on contract which later became 19 months because I had called the customer care for 1 month notice and not written to them. Later they advised that my last bill would be £15. But surprisingly I was billed for £94.75. I was never billed so much before, and especially because it was te last bill, I made sure that I dont use it much. I have serious doubts about the correctness of billing. When I tried to login, my account was deactivated. Horrible customer experience. There was no problem when I was with the network, things went wrong when I decided to move to another network. But I am very sure Asda mobile is bad to the extent that I sometimes think they may be fraud.