Folks have all kinds of different reactions to various drugs. They are what you might call statistical outliers as the vast majority of the population is going to have a fairly text-book reaction to most commonly used drugs. Literally anything is possible, ranging from euphoria and fun to outright death by anaphylaxis due to innate sensitivity.
An anecdote: My mother has received Opioid painkillers maybe 3-4 times in her life, with 75% of those experiences following the birth of myself and my siblings. She received a dose of Morphine after I was born, a standard dose and it caused intense dysphoria, fear and itching that was so bad that she removed a lot of the skin from her face and was left with severe irritation. She doesn't understand how anybody could enjoy Opioids, but it's different strokes for different folks. My mother would be one of those statistical outliers I mentioned.
Now, a lot of people have been ingrained with a large amount of fear regarding Opioids. We encounter people on the forum who are afraid of dying by respiratory depression after taking a 7.5mg Hydrocodone (Vicodin). This is more of a psychosomatic issue, but we do encounter it.
OP, do you have any "hidden" reservations regarding Opioids that could be causing this anxiety or do you feel that it's directly a result of taking the Codeine?