The letter that the IRS sent you should have explained what they changed.
If you were still married at the end of 2021, you can't file as "Single". You would have probably filed as "Married Filing Separately".
If you had health insurance through the Healthcare Marketplace and received Advance credit to reduce the cost, filing as Married Filing Separately disqualifies the credit. Part of that $1500 could be related to that.
Did you have ANY income? Or lived in a Community Property State and had to include some of your husband's income?
It is extremely difficult to give you a complete answer without seeing and knowing the entire situation. I suspect that bringing your situation to a tax professional is the best way to get the answers that you are looking for.
