Where exactly does it say Pending? If on TT, that's because the program has no way of knowing that you actually mailed it and got your refund. But if on the IRS website, it may be that the government is issuing you a refund for the new unemployment comp deduction. (But why would you be checking the IRS WMR when you already your refund?)
Or is it this, maybe: The IRS is sending out Stimulus #3 payments in batches, and will take a few weeks to complete them all. This one comes to you separately (and automatically), not as part of your 2020 tax refund. You can check the status here:
@SweetieJean is correct.
Was there some reason you may have accidentally tried to e-file the tax returns? Since the returns were already mailed in and the refund was received from the federal, then the e-filing would be rejected. The social security numbers would already be on record at the IRS as filed.
You haven't mentioned if the state was received. To contact your state, see: How do I track my state refund?