They sent you a refund? Did you enter any 1099R? If 1040 line 4a is a lot less than 4b (which should be ok) the IRS has been lowering the taxable amount on 4b and refunding you. But then they will eventually want it back.
Did you make estimated payments and forget to enter them?
If you made estimated payments see if they are on line 17. Sometimes they end up line 16 with witholding. If that is the case the IRS will eventually see line 16 is wrong and ask for it back.
