Questions about Data overages for the iPhone 5 (Verizon)?

[AWalktoRemember]

I just upgraded to the iPhone 5 on Friday and Verizon made me give up my unlimited data plan in exchange for a 2 GB plan. They said based on my usage in the past, it was unlikely that I would go over the 2 GB often, so that's why I chose it rather than the 3 GB plan (or whatever the next lowest one was).

However, two days later (today) I get a succession of emails. First email this morning said I was at 50% of my data for the month. The 2nd email I got later this morning said I was at 90%, and then I got the 3rd email early this afternoon saying I was over my limit and would incur a penalty cost. For reference, my cycle ends on the 26th of this month.

My question has 2 parts:

1. If I just upgraded on Friday, shouldn't my unlimited data plan have been in effect until then? They shouldn't be able to go back and calculate my data usage for the entire month, right? I was going under the assumption that, starting Friday, I would start having to watch my data usage. I couldn't possibly have gone over if that's the case.

2. What programs / apps use the most data? I have the basics on my phone, like FB, Twitter, Instagram which I check frequently, but I rarely send / receive messages on any of them. I upload photos to Instagram maybe once a week. I also have numerous games, of which I usually only play 1 (Temple Run). I don't watch many videos and I hardly ever stream music (I use my own library).

Thanks in advance for your responses!