Most likely a carrier issue or network congestion.
What model iPhone and what carrier, speeds you're getting and location.
Looks like AT&T has throttled my data since I reached 5 GB.
Thanks for the replies.
That explains it.
They didn't send you a text message?
That sucks. One good thing about unlimited on Verizon is they aren't allowed to throttle speed on 4g by some law. It's great since I use an average of 10gb a month
Just wait til you upgrade and they yank your unlimited data plan....