Thankyou so much for answering my question :)
The hydrocodone is unlikely to related to Down Syndrome (Trisomy 21, or three copies of chromosome 21): it has not been reported to cause chromosomal abnormalities. It is not certain at this point that your baby has Trisomy 21. That is the reason you are having the amniocentesis, which will test for the number of chromosomes directly.
You should continue discussing your use of hydrocodone with your Obstetrician. The highest risk for continued use is that your baby may have withdrawal symptoms after birth, but these can usually be easily treated. Decreasing the amount of hydrocodone during the pregnancy should not cause your baby any problems.