Mood swings is not the correct term for this seeing as the chemicals in our brain are quite physically being altered.
I often relate it to being given a list and being told that you MUST get it done OR ELSE and you do NOT want to know what or else is- except you can't finish the list because of matters out of your control so you become stressed out and almost feel betrayed.
Its hard to explain but I dont think mood swings would be the correct termology seeing as A. Its not caused by our doing, mood swings are ussually caused by the person being "triggered" by something that makes them upset personally when regularly it wouldn't upset someone.
For example I think about being hangry and getting upset over something that is simple because of that and then immediately becoming happier because the body is fed, making the hormones and chemicals in your brain happy right? This is something that can be changed quickly you know? Or that is unpredictable.
But Rejection sensitivity is ussually caused by other peoples action and we know its ussually irrational. Its our brains being dysregulated because the plan/ routine was disrupted, like when an autistic person cant find their shoe and now are running late, messing up their entire schedule.
I can only explain it as an intense feeling of disappointment, sometimes if caused by someone close to you it will feel personal despite the fact you can mentally think "thats silly to be upset about" but you just... cant stop.
Sometimes it can last hours and makes you feel very isolated and alone, it makes you sometimes have panic attacks too.
The absolute worst thing you could do is make fun of us for it because we KNOW it's stupid but we cant stop being upset. We are upset at ourselves for being upset, so mocking us would make it worse and feel more alone.
Logan does a good job as an example here, figuring out what the main problem is, giving him a bit of space, but not letting him feel alone.
A mood swing (to me) would be if you wanted a cookie but someone ate the last one, getting upset, only to realize there is more, now feeling better because the issue is solved.
With rejection sensitivity, it's like going to the same cookie shop every week to get the same cookie and talk to the same people. A routine. Well you had to stay late for work and now the cookie place is closed. You can still get a different cookie, hell, you can get a cookie that's almost identical, the problem is still not solved. Providing a replacement item sometimes helps but ussually its the routine being disrupted that causes the "break down" not the absence of the cookie.
OR another example is like your partner/parent saying that pizza is for dinner and then you come home to tacos. You still like tacos but your brain is now disrupted because you already mentally prepared for and looked forward to pizza. This makes me very sensitive. Don't tell me you're making pasta at 8 am and then I come home at 8 pm to tacos UNLESS you call me a few hours in advance so I can register it with my mind. God that sounds so pathetic. "Yeah I cry if I get tacos instead of pizza except it actually has nothing to do with the food at all and its the idea of the routine/ list being ruined"
I hope this dosn't sound like im scolding anyone or offensive either, but they are definitely different to me.