I had a $29 dollar plan , The first bill was $72 (dont remeber exactly) and it says that the first bill can be higher than the others . The second bill was for $218 and now the third one is for $200 . It doesn't make any sense to me when it says extra airtime usage cz whensoever I check I can see the 100 mint usage is full ??????
If you take a look at your actual ebill, it lists all the charges and what they're from. You need to take a look at your ebills which can be accessed via self-serve. Your bills are high probably from some type of overage; most likely data in this case. Have you been using apps like fb/twitter, etc and the internet?
Look at the 3 page of your bill. You usage details are there so you'd know exactly what you used (minutes,data etc.) that pushed your bill up. Take a screenshot, if you want, and post it here. We can tell you what happened if you still need help. Based on what you are saying I hazard to guess you went over your minutes and you paying overage.