" I engaged in some behaviors that violated my own moral code"
The above is actually the most important part of the post, IMO.
The cough syrup was just a desperate attempt to medicate those feelings away (by collapsing into sleep and fleeing emotion). Cough syrup was one of the only choices you could think of to get that relief without relapsing (and I am SO glad that you did not drink).
But yeah - cough syrup (which BTW, many addicts do incorporate into their drug regime...) or too much Valerian or any herb or medicine which can be justified...it still is the same impulse to forget, the same reaching for oblivion, but without the drugs or drink to really get there.
So, the core of your experience and post is in breaking your moral code and feeling shame. This is what you'll need to work through and create boundaries about, because the next time you do it, or the next time, it will likely precipitate full on relapse.
PS. My experience is that I have relational and sexual behaviors which I am shamed by. I'm no moralist, and I believe in very open expression of self through sexuality, but - mostly in the area of people who should be off limits to me sexually for various reasons - I have crossed my own lines at times. I have given this behavior up in my sobriety. I had to also give up internet connections with romantic or sexual possibilities, and dating in general - until my recovery is solid AND I've had time to work on and understand those behaviors which I practice but then feel shame over.
My suggestion is to add whatever behavior let to the Nyquil to your sobriety. Quit it. Set a date for yourself until which you will not engage in this behavior again. Then set yourself toward figuring out its origin, purpose and meaning in your life.
We are addicts. In sobriety, our issues re-emerge in many forms - gambling, porn, anger, infidelity, on and on. We can't hold guilt and shame any longer, because they make us drink to forget.
I believe that the work we do across all the areas of our lives is our recovery. I read somewhere in the beginning (paraphrasing here) - that the trick to quitting alcohol is to build a life so satisfying and right for you that you don't have to drink over it.
I think about that a lot.