Here's the bottom lime: you're absolutely going to need an attorney to represent you.
Can Nyquil and Benadryl cause you to "test positive"? Yes, but only for the substances in them. They do not contain controlled substances, so assuming that's all you ingested, the lab test will come back negative for controlled substances and the under the influence charge won't be an issue.
Will you be charged with DUI? That's another story. You can be charged with DUI if you are driving under the influence of drugs (illegal, prescription or over the counter) if your driving was impaired by whatever substance you ingested.
Your attorney can analyze this scenario, including the search issue when they checked your physical symptoms and advise you from there.
You can be charged for DUI even if no alcohol is in your system. Taking nyquil and and benadryl might make you impaired enough to charge you with 23152(a) VC. Despite the fact that they are over the counter medications, you cannot drive if they impair your ability.
You need an attorney to deal with this. Get one ASAP!
Legal disclaimer: You can learn more about me by clicking on my Avvo profile. Disclaimer: Please note that this answer does not constitute legal advice. It is merely intended to provide general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.
You might have traces of the medications in your system. Most crime labs will test for the presence of these susbtances, not the amount. Once they have a positive test, they will prosecute. It will be up to you and your attorney to get part of the blood sample retested by your own lab to test for the amount, so you can show it was not enough to impair your ability to drive. Get an experienced DUI attorney immediately.
Sign up to receive a 3-part series of useful information and legal advice about DUIs.