I am not sure what country you are in but I have never heard anyone be diagnosed by a questionnaire. It can take months and even years of weekly psychiatry to form a clinical diagnosis. This is shocking negligence. I feel bad that this was your experience. I'm so sorry. If that's how you've been diagnosed I would almost certainly look for a second opinion. This is not how you get diagnosed.
I have had three diagnosis, BPD, Bipolar II and C-PTSD. Oh, and EDNOS but I sort of class that as something separate to my mental health diagnosis which I probably shouldn't but hey ho.
I'll focus on the BPD first. For me it is extreme, intense emotions that can flick like a switch. When this happens it's like I'm possessed. It's obviously me but it's nothing like my 'usual' self. It truly feels like a totally different person. I sometimes even feel like I don't look like myself.
When I switch I am the most vile, hurtful, toxic, spiteful human being and so incredibly angry at the world and everyone in it. This switch then flicks back at some point to a loving, kind, generous, relaxed Emily. I don't know what makes me switch, I've never managed to figure out my triggers and I don't know what suddenly stops these rages either. My rages usually last 3-4 days for me at the moment. When I come out the other side I often feel immense guilt and then spend the next week apologising to people. I also have a Favourite Person which is extremely common for those with BPD. Google that for more information if you haven't heard of it. It's incredible that this human being has stuck by me for so long. I'm incredibly fortunate.
My Bipolar manifests not in mood swings like the BPD, but hypomanic episodes followed by crushing depressions. If you've ever watched the film 'Limitless' that's the only way I have been able to describe to people what being hypomanic feels like for me. I have type 2 so never get to the point of full blown mania with delusional thinking or psychosis. That's only ever happened once and was over 20 years ago. Hypomanic feels great to begin with but when you are 2 weeks in, living on a few hours sleep, think you are some sort of super human, can't stop doing things day and night like DIY at 3am and cleaning floors with a toothbrush, it starts to really deteriorate your sanity and you also know in the back of your mind that what goes up, must come down and you will inevitably crash. I've found the longer I am high for, the bigger the depressive crash.