How does data usage work? Having some problems?

I've had a Droid Pro for 2-3 months now, Verizon, unlimited texting, shared minutes plan with the family, and a 2 GB data plan. The first 2 months or so, I had absolutely no problems with the data plan limit. This month, though, the MyVerizon app is telling me that I've almost reached my 2 GB limit and I still have ~15 days before the cycle ends. What's bothering me is that I virtually have not changed by usage since those first two months, and I want to know why it's racking up now.

I'm using the home Wi-Fi most of the time. The only apps I have are facebook, tumblr, and bakery story. I might check them once or twice a day, but I'm not spending a gross amount of time on them. I text a lot, which I didn't think would impact it all, but I do have a friend with an iPhone who sent me 5 to 10 photos while she was on vacation. Not sure how much that changed things. I accidentally let a pdf download a week or two ago, but it didn't show a huge increase in my usage when it happened.

Yesterday I was at 50% used up, today I'm at 75% and have hardly used the phone outside of texting. What am I doing and how can I stop it?

Photo messages use data, and the apps you use could be using data w/o you using them. And txtinq does not impact that at all. You could turn the auto sync of the appd off so they won't use data without u using them.