Sinkhole!?!?!? I filled a posthole a couple weeks ago and presently the hole...

I filled a posthole a couple weeks ago and presently the hole is back and it's deeper and larger.
I don't hold any hot river getting into...
Answers:    Most of your answers are right ones so far. Best guess is some kind of blankness underneath for any of the reasons the other answers stated.
My request for information is, what did you fill the post hole near? Did you tamp it down?
Can you take an iron rod longer than the hole be to begin next to, and push it down beyond that depth? If, so you got thoroughly loose soil below it and a potential problem.
Immediate fix would be to dig out a small portion of the current hole, pour within some instant concrete mix (cheap), let it set up, afterwards put the dug out soil back on top. If soon the concrete itself begin to sink, you better get someone from the city or county extension bureau out to inspect and offer some resolution to doesn`t matter what problem they think it may be. And, similar to one other answer, better hope it isn't the top of a sink hole.
How do you take the fliters out of...
And China is P.O'd at you