I see that I completely stop tolerating melatonin, I even get in a bad mood when taking it and feel sleepy all day, how strange when before it was the opposite and I was even more relaxed,
ME changes over time. I've had foods that I've tolerated well, then became intolerant to, then tolerated again. I've had treatments that worked well the first few doses, but then stopped working and never worked again. The body is a complex bunch of interlinked processes, and if one changes for some reason, it may affect others.
I simply expect my ME to change, and I treat the negative changes as just more hurdles to overcome or get used to.
You may regain tolerance for melatonin in the future. You'll have to retest it once in a while to see what your response is.