Did my iphone chew up it's credit by using the internet?

I know this may sound like a stupid question, but i was suppose to have around $70 left in my phone, and now i have nothing! I think i used the internet ONCE from the day i topped it up. I have wireless running around the house (i pay $50 a month for that,) and an 3g ipad 2, macbook, ipod touch and iphone 4s left lying about. Is it possible that somehow my iphone used it's credit by me being on my macbook or ipad or something? I can't work it out. All i know is that i'm suppose to have $70 and i have ZERO. I'm also on the plan where your credit doesn't get chewed up until exactly ONE year after you've topped it up. I'm trying to think what i could've done. I didn't buy any apps, except on my ipad, and my itunes vouchers were used for that purpose.

Yes, it did…