It can be kind of hard to tell between having a meltdown or just being exhausted. I am autistic as well, and I’m trying to educate others through my personal experience with autism. Plus, I have ADHD, bipolar, and anxiety as well. It’s a handful, that’s for sure. Here’s what my take on your question is: A meltdown is usually like a panic attack that gets further than it should. I have experienced the exhausted part before without having a meltdown. Sometimes I am completely calm, but I end up feeling “too calm”. When I get too calm, that’s usually when depression starts to hit me for a short time. A meltdown may occur at any time, like a panic attack. It can even be about absolutely nothing, and you just have a feeling to cry. I’ve had that happen multiple times before I checked your profile, and I see you are older than me, but if you have any questions about autism or need an extra person to talk to, my dms are open. I’ll just have to remember to check them on here often. Otherwise, you can find me on Instagram. I check that more often. @bopitmaster8 is my username. Oh, by the way. Nice to meet you, Nico. The name on here is not my real name. The name on here is actually the name of my Guinea pig! 😂 My real name is Cain. Hope my advice helps! :)

Exhaustion will lead to a meltdown. but when in meltdown you may experience exhaustion too and other things. The other things may be like going nonverbal, stimming in any way wether its moving physically or maybe repeating words to help soothe yourself consciously or subconsciously. Now this is limited to what i know of myself and what i hear from others. You may have meltdowns with patterns particularly to you. Doing research and being aware of what you do and how you react when sad or overwhelmed may help you differentiate exhaustion and meltdown too.