You can go on this site and go on My Verizon. Then you can look up all your usages (text,data and phone) and see exactly what you are being charged for. You can also look up past months. Are you getting charged for data? Are you going over you allotted minutes? Are you going over your allotted texts? Print it out so you have something to look at then call CS back if there are errors. Also maybe you are not sure of your bill date. It may end before the month ends. My bill is due the 20th of each month even though my billing cycle ends on the 25th and both those dates are before the month ends.
Also be sure noone else has access to your phone or if you are on a family plan that noone else is going over their minutes.