I change my alternator in my 99 cavalier now my check engine light is on what could it be

Check all the wires. Make sure you hooked everything back up. Take a volt meter and test the output side of your new alternator. Make sure you are getting 14 volts or higher. Rebuilt alternators are sometimes bad. Also put the volt meter on your battery. See if it is putting out at least 12 volts. If less, your battery may have been bad the whole time. If all looks well, bring to nearest Autozone and have the codes read. At least it will cut out all the guess work.