parasites, heavy metals, mold, and toxins that silently build up in the body
It can be any of those things (though “heavy metals” is very unlikely these days unless you live in a slum which still uses lead piping or is built on a toxic waste dump). It can also be (and more likely is) stress, which can come from any number of places in your life, poor ventilation, poor hydration, nutritional deficiencies, unusual sensitivities, and/or some undiagnosed chronic illness. (If it’s short term, some of this could also be burnout, or your immune system fighting off a disease which isn’t showing any obvious symptoms.)
Aside from some of the rarer chronic illnesses, there are treatments or cures for all of these, even if it’s just “remove yourself from the environment” (which sadly may not always practical). Out of the usual quackeries, the only thing which sometimes works is certain forms of “detox cleanse” treatment… but mostly because the people who are doing them are chronically mildly dehydrated and/or have a diet deficient in certain vitamins, and that particular form of detox treatment provides what they’re missing. Any actual “detox” which might come from this is due to temporarily restoring efficient liver and kidney function, not from directly removing “poisons” and/or “heavy metals”, if that’s even one of the issues (it usually isn’t).