"We really don't know if out ancestors, never had troubles with something like a genuine POIS" - If they experienced such a consistent condition, it would probably be documented, even in this narrow, rare extent of cases.
"the fact that it might be more frequent now, could be due the factors that have led to increased number of allergies, or immune disorders, in the last decades: pollution, unhealthy style of life, etc." - Yes, I agree, as well as the consequences of oversexualized age leading to abuse of pleasure related activities.
Hard to tell, where the truth is, but I still think, it has lot to do with addiction/dependence, or we should try to accept, there are more arousal/orgasm-induced conditions sharing similar attributes, but being different in their causes (porn-addiction, post-coital headaches, POIS, sexual exhaustion, etc.).
"It wasn't necessary to be addicted even to masturbation to trigger the entire set of symptoms, as it happened to me." - Many people on yourbrainonporn describe an outburst of symptoms after being aroused, after thinking about sexual content or experiencing non-coital/orgasmic sexual stimulation.
Autoimmune diseases are quite broad in their clinical profiles, almost every existing symptom could be attributed to them. Anyway, I am going to abstain (even from sexual fantasies) as long as possible and take a complex bloodwork, EEG, MRI etc., to find out more about my condition - maybe I suffer from other, undiagnosed and probably severe disease, maybe not.