Weldome to the community!
Are you sure that bill you received was your first? If you activated your line on October 14th , your first bill should have been generated shortly after that date. Your billing cycle is set based on when you activated your line (see here). As well, for any given billing cycle, you are charged the upcoming monthly fees as well as any overages you might have incurred during your previous month. So you are charged in advance for your monthly plan plus any usage above your allotments previous month. Since most billing cycles start within a few days after activation, first bills usually include the upcoming monthly fees as well as prorated charges for the few days before your billing cycle started.
If you just received a bill, is it possible that is your second bill? If it includes charges for your first bill as well as your second, that could also account for the increased amount.
Have you had a chance to view the billing details on your My Account? You should be able to Save and/or Print a copy of your bill there.
You should note that this forum is community-driven and not intended as a venue for customer services. We do not have access to customers' accounts. If you would like to discuss your bill, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀