What can I do?

I have legal custody of my child. I allowed my child to stay with my ex-mother-in-law last school year due to financial issues I was dealing with. We had a notorized agreement that stated he would stay with her for the school year and I would be picking him up this summer. Well, when I called her to inform her that I was coming to pick up my son, she was agreeable. That was on August 02 of this year, on the 3rd she went to a lawyer and on the 6th I was served with papers stating that she was filing for emergency intervention because I was planning at that time to move to another county. My papers stated that we would be going to court August 31st, however, on the 12th, I got a letter stating that the court date had been cancelled because the judge already granted the intervention and awarded her temp custody until we go to court.
How is that fair to me that I get served the papers on the 6th and she gets the intervention on the 7th? Is there anything I can do to fight this? I know that I have to go to court and will be making arrangements with their lawyers (my ex husband is filing for custody and his mother is filing for custody in case the lawyer finds him unfit and if she can prove me unfit). Any and all help would be greatly appreciated.
*Edited to add...I live in Arkansas.*

You had no authority to leave the child with the mother in law. If the court order did not specify, your signing notarized statements agreeing to this or that is not legal. You are/were, in essence, in violation of the court order. Even if you had custody, you did not have permission of the court to make alternate custody arrangements. Your spouse, ex, or his mother capitalized on this and that is how they were granted temporary custody ( My best guess as to why).

The fact you were unable to care for the child is going to be difficult for you to overcome. The court may view it as in the child's best interest to remain where he was, where you left him. He isn't a puppy to be moved around---he is a human being that needs stability. This is not a criticism of you--- a lot of people fall on hard times. But not everyone leaves their children with someone else---even if the move was clearly in their best interest. You left the child where he could be cared for--that is to your credit. But the fact you needed to leave him in the first place is the hurdle you will now have to overcome if you are to get him back.

As a widow with seven children, I do understand your struggle. I wish you the best.

Good luck.