If you're looking for answers, then start recording your mood, any sort of triggers, the amount of sleep you got in the night, medications. It can help teach you more about yourself, what causes that. The line between Bordeline/Cyclothymia Borderline/Bipolar II is so so small, and a lot of Borderlines also so have some sort of bipolar disorder. The thing I've seen time over time over though, is that there is a pattern or some sort of cyclic pattern to bipolar/cyclothymic mood swings and they happen for no obvious reasosn, wheras borderline mood swings are triggered, and there doesn't seem to be a pattern.
Cyclothymia is a form of bipolar, BTW, just in case you didn't know.
Ranting is healthy, so don't worry about it.
People deny that mental illness exists, and it's generally best to just ignore them. They are not doctors, they are not psychiatrists and they most likely have not spent years studying illness, and therefore, they have absolutely no right to comment, and mental illness could hit them any day and it could be anyone of them who ends up in a psychiatric word for murderous-psychotic delusions, and they bigheaded attitude isn't going to prevent that.
BPD does exist. If you think you have it, then you need to see a Pdoc. BPD is hard to diagnose, a lot of people will always advise you to get a second opinion if one psychiatrists diagnoses you with it.