Did my rate go up? Can I change my plan?

It appears my plan rate will increase by over $70 as reflected on my bill for January 2016.  Granted, I was accidentally 6 days late paying for December but it usually would just double December's rate and add it onto January's. I paid the amount I would've paid for December but the remaining amount for January is significantly higher than the usual monthly rate for the last year. Has my rate changed or is there a late fee I was unaware of? Lastly, will I be able to change my plan as I can't afford the increase if it's permanent. Thank you!

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