Hello,
When taking Phentermine (or any other medication for that matter) it is very important that you do not routinely use other medications, especially cold preparations.
I would suggest you should decide which you require more at this point, cold medication or the Phentermine, but do not take both together In fact, if at all possible, I not take either today or tomorrow.
Your symptoms may or may not be connected. As you have a cold/flu your symptoms could be related to that verus the medications. This is based on the fact that you took the Nyquil last night and just began feeling strange today. You would have had a reaction must quicker if it were the combination.
If you have any other questions, please ask.
http://www.mayoclinic.com/health/influenza/DS00081
http://www.mayoclinic.com/health/flu/FU99999
http://www.mayoclinic.com/health/flu/QZ00072
http://www.mayoclinic.com/health/flu/QZ00072