This sounds odd..Like its not seeing you have a data connection. Does it show an E/3G next to your carrier at the top?
You could try flipping airplane mode off/on.
I am on the same plan I have been for 10 years, didn't change that at all. I am actually still having the same problem too since I was going to try a hard reboot and I still cant wear the dang battery out on the thing LOL so as of yet, no fix