I have a Galaxy S3 with a gophone plan and I get this same exact message when I am down to less than .5gig.
Sometimes, I can restart my phone and it will work normally. Today, though, I couldn't get it to accept my request for data, so I added another gig to my account. Had to restart my phone again before it allowed me online. I don't know why it does it. It has happened once or twice before, but not every month. Very weird..I have no restrictions on my account, and being pepaid, you'd think you'd go the whole allottment at which point the service would THEN shutdown and not allow connection until you added data. If I have a balance of data left, I want to use it, not waste it before my plan renews!