SOME PEOPLE who are more "susceptible" do get the symptoms you describe. I treated youngsters in The Priory who has such severe adverse reactions like you did…. it DOES happen in some people.
No drug is EVER safe, especially illegal ones! some people have a reaction to relatively "safe" drugs / medication like aspirin…. its just shows that no drug is universally safe.
What exactly is your question?
You do not have dementia, you are probably perfectly healthy.
Are you asking why you had the reaction you had?
Firstly the marijuana could have been laced with another drug and that might account for part of your reaction. if you were drinking alcohol or if you take benzodiazepines, barbiturates, hypnotics, antihistamines, or certain other drugs (legally or illegally) that may have exacerbated the effects of marijuana.
Having said that you have the answer, "people have different reactions."
Smoking marijuana is often not like it is in a film and although most people feel relaxed and have other pleasant effects that certainly is not universal.
I remember when I tired marijuana I felt calm, I was laughing, and I became dangerously suggestible. if someone would have told me to run into traffic to get hit by a car there is a good chance I would have done it. I was lucky to be with friends who would not let any harm come to me. However they did screw with me and convinced they me I was in Buffalo, Canada (there is no such place) and then I was convinced I was in Buffalo, New York after walking one block.
It is was a very strange experience and not consistent with typical reactions to marijuana but that is what people risk- some will have a terrible or at least a very odd experience that may be closer to LSD intoxication or barbiturate intoxication.
But alcohol also creates a wide rage of reactions- some people become stimulated, active, social, and fun. some feel relaxed, sedated, and at ease with life. some become sad, agitated, and depressed. some people become angry, rageful, and even violent. A small majority actually become suicidal.
Alcohol has also been conclusively shown to be far more dangerous than marijuana. Alcohol is toxic and overuse can destroy the body more so than any other drug. Alcohol can lead to physical dependency which makes it much harder to stop and once that sets in people can't stop or they can potentially die (without treatment the mortality rate from withdrawal is up to 25%). Marijuana has never killed a person but alcohol has killed thousands and it can kill by drinking or by not drinking (in the physically dependent). The number of deaths caused by secondary complications of alcohol is staggering.
Alcohol, where it invented recently, would be a highly controlled substance. I can also tell you if I had to become addicted to a drug the last one I would pick is alcohol.