Insurance and bills HELP!

I have no idea what other group to ask this about. Okay so I was on medicaid all through 2018 up until 2019. Both of my kids were seeing a pediatrician who had my insurance info during the whole visit. Now I remember one time before we got on our insurance my kids had lab work done. So the office said I owed 998 and I thought it was from the lab work, well I never get on the portal but I did today and I see a balance for my youngest of 1300!!!! Apparently their was a schedule pay plan for 998!! And it was for her vaccines and visits during 2018 all when They were on insurance. I look and see not any of the payments have been adjusted by the insurance. Wtf! I feel like a dumb ass for not looking at the portal but I thought this whole time they were sending it to my insurance at every visit... now I am not on medicaid anymore and on a new plan. Wth can I do? I do not want to pay this when I had insurance at the time.... is their anything I can do??

Thank you.