Is it possible to change my data plan back to pay per traffic now?
No problem. 
I’d changed your data plan to PPT (Pay Per Traffic) and reset your changable date too. If you’d like to change it back I think you can do again.
In general, you can’t change it again for 1 month
https://mudfish.net/docs/en/policy/data_plans.html#how-to-change
That’s helped a lot, Thanks!
Cool! I hope now everything goes well with mudfish. Please let me know if there are any other problems.