you can be on a medication for a looong time even and still have a bad reaction. Your body is in a constant state of change, and your metabolism can be altered by some very simple, basic things, sometimes. If you become dehydrated, it can affect medication levels in your body. If you are fighting off a virus, cold, flu, whatever, it can affect your metabolism. How muuch you have eaten (or not eaten) recently can affect your metabolism or how a medication works or does not work in your body.
SO many things can affect metabolism, so many things can affect how medications work, so many things can affect your physical and mental health, and they are all interelated. Even something simple like the temp of your body, which can be altered by environment, activity level, wellness state etc.
ALso, most people do not like to worry their loved ones, so many often do try to not worry loved ones, even if they are aware something is not quite right, altho often we can experience some things that we convince ourself are just not real important or serious.
And in cases of psychosis, or serious depression etc, it is common to not voice those things due to embarrassment. People do not like to think anyone else might think they are "crazy" and people also do not like to share their own dark feelings very often. When we do share our dark feelings, some people try to minimize them, some try to get us to shrug it off, some try to tell us to buck up.....sometimes people shy away if we vocalize our dark feelings, so we often try to keep them to ourself.