Health insurance use- is this wrong

I have health insurance for my child. My ex went out last year and got insurance also. When she takes the child to hospital, Dr, etc.. they normally bill both carriers. Mine always pays first so when her insurance pays it shows an overpayment made on account so hospital sends it back to insurance company and they refund the ex. I could be guessing but I think they think she over paid her amount and don't realize that another insurance company pays. My court order says I have to pay 50 percent what insurance doesn't cover so not only does she get paid from her insurance company I pay her 50 percent of what my insurance doesn't pay. So I think my ex benefits by taking my child to hospital or Dr's. She's making money. The reason I know is I checked child's records and seen that hospital sent $1100 back to insurance company saying the account was paid. I've noticed my kid has been to Dr's alot in last year for minor things such as ER visit with a 98.4 temp. I suspect foul play so is there anything I can do or should do? Or is this legal what she is doing? Oh hospital bills both as primary which I think is weird.