What does Cellular Data Usage Current Period mean?

I have an iphone 5c and Unlimited calls, unlimited texts and 6GB of data. In cellular in settings it says Current Period: 8.4GB and Current Period Roaming: 0 bytes, am I going over my bill?!

As 8.4GB is more that 6GB - yes, you are over

of course, that also assumes that you set your current period start / stop dates correctly (it does not set them itself - you have to set them), but even os - sounds like you would be over anyway.

If that reading is accurate, you have exceeded your data limit for this billing period.