Medical group charging me $1500 out of the blue

I attended an intensive outpatient patient in Nov 2023- Jan 2024. I paid $1200 for the classes. I was told they were all paid for and that if I attended or not nothing would change.

This week I got a bill in the mail for $1500 for all the classes I took in the new year. I was NEVER told I would have to pay anything more than I originally did.

I was also under the impression that I paid out of pocket instead of going through insurance. But the "itemized bill" they sent says I did go through insurance. $999 for the outpatient, $150 copay, $150 early out vendor fee $617 and $216 for other insurance mumbo jumbo. After all of that the balance ends up being $150 for the 11 classes I took during the month of July.

I don't know what to do about it. Can they force me to pay if they never informed me about it? Do I fight the insurance company or the health company? I'm confused and angry. I can't afford $1500 out of nowhere. Any advice would be appreciated.