I'm gonna sound like sort of a hypocrite, but I self diagnosed. I knew all my problems stemmed from trauma, and I talked to one counselor about it and she said I definitely fit the definition for ptsd, but when she asked me to see a psychiatrist to confirm it, I politely refused. I was 15 and I know for a fact my step mom would put me on more drugs to shut me up just like she did when I was officially diagnosed with aspergers. Knowing what zoloft did to me, which was nothing short of morbid bouts of depression and suicidal ideation non stop. I didn't know what to do so basically I looked online for any self help tips. But nexterm month I'm actually seeing a psychiatrist for the first time in 5 years, since I'm 22 now and have all the power to refuse prescriptions. Plus my wife helped me realize I have serious problems and I need to deal with them.
But I think self diagnosis is ok, but for only a few reasons. That is you find your symptoms, and you find a way to deal with them like separate problems. If the symptoms persist, you look into legitimate resources to see if you fit the criteria of any mentail disorder so you can find coping methods for what you might have. If that persists, you see a doctor or psychiatrist for a diagnosis as well as treatment and any services you may need.
If you just had a few flashbacks (I'm sure tons of people have had them on rare occasions) doesn't mean you have ptsd. So if you go on social media and say you have ptsd because of one flashback, or your girlfriend dumped you, not only are you making yourself look stupid, but you're invalidating people with real mental disorders by making the mental disorder look like it's not really that bad, when in reality were in a living hell, and some of us are affected to the point were disabled. So it can be a good and a bad thing.