Shannon Mills, 19, was using Facebook and video call service Facetime while sitting round the pool on a family trip to Turkey.
She thought it was fine after agreeing to pay O2 £120 for 30MB of data to use in the Mediterranean country before she left.
But she soon started to receive messages from the phone operator warning her of spiralling costs.
She was first told she owed £2,808.

O2 then blocked all data use from her phone but the big shock was still to come when she found out about the total bill of nearly £22,000.
She said: "I thought that was it. I had a big phone bill but I'd deal with it when I got home.
"It wasn't until when we were relaxing at the pool waiting to leave my dad said I should check again just to make sure. I did and I couldn't believe what I was seeing.
"I couldn't use any data after they'd said they blocked it so I don't know where the bills were coming from."
However, a spokeswoman for O2 said the customer had asked for limits to be removed from her account.
The spokeswoman said the company was prepared to only charge Ms Mills for the first 24 hours of her trip meaning she owed £2,842.

bbabo.Net