So, 2 days ago I noticed there's some sort of cut in my palate, took a photo of it https://imgur.com/4X96mCa
Any idea what is that? is that the sign that my upper jaw is expanding?

It's hard to tell if this is a normal feature of your palatal rugae. It's not uncommon to have a demarcation down the middle of your palate like this:

But I take it from your post that this is new? Does it hurt? Is it bleeding? Can you tell if the oral mucosa is actually lacerated? If so, I suspect it should heal relatively quickly if you prevent recurrent trauma. Tell us more about your routine and/or what you think could have caused this. Tongue posture or even an expander used at a reasonable rate shouldn't cause a tear.
It doesn't hurt and neither it's bleeding. Well, I can't really tell, but when touching it with my tongue it feels like a straight, but not so wide hole. An interesting thing to point out is that when I wake up in the morning there's almost nothing there (there's still a cut, but not so big). The thing that caused this is my recent improvement in "mewing" I guess. Last 3 month I've been sucking my tongue up on my palate without actually pushing it and a week ago I read that I should be pushing with my tongue, not sucking on my palate. Since I made that change, I've noticed that my side with asymmetry is getting pushed out and it's starting to widen itself, which is indeed the thing it should be doing. Should I slow down on pushing during the day? I honestly feel like I could be pushing it way more than I'm doing it right now, but maybe since I just started pushing it's little too much for me. Do you think that pushing is the problem?
If you poke at it with your finger nail is it tender? Does the mucosal surface seem to be torn? It seems like the fissure might not be a laceration but just a shifting of your palatal rugae as your midpalatal suture separates. It is curious, but I guess I wouldn't worry about it if there's no pain or bleeding. If pushing with your tongue seems to be helping your symmetry and expanding your palate, I would keep doing what you're doing.
When I poke at it with my fingernail it feels just like the rest of the palate, not soft but not too hard.
I don't really know if it's torn but I took a better picture, it looks like it's recovering to me, but hey I'm not an expert haha
I will keep on pushing then! Thanks!