After watching myself playing, i realize i do something weird with my thumb when i strum. Basically i bend and release the tip a lot, like this:

 

http://imgur.com/a/SpuRB

 

You can see it while i'm playing i think: 

 

4 months ago i started to have pain in my right wrist and i couldn't play for a couple of months, maybe that's the reason ?

anyway, my question is: what am i doing ? and should i stop ?

Just speaking from my novice experience with guitar many years ago before I took up bass. I used to use a plec and would do similar if I was using a hard plec as opposed to the soft shark fins, it was kind of a subconscious "search for the sweet spot" when gripping and I had pain too from gripping that wee bit too hard. Try a more flexible plec' perhaps? The pain sounds like a bit of Carpal tunnel so warm up exercises would always be beneficial... though I'm not great at even following my own advice  :unsure: I often get a gammy hand from lack of warming up.

I think I might do the same thing.  I have hitchhiker's thumb, so if I place my thumb on the back of the neck, the end of my thumb is at a right angle to the neck and placed under a lot of stress.  So, I think I grab the neck with my palm with my thumb over the top, like you, as a way to create a brace for the pressure I want to apply to the strings.  If I try to keep that thumb pressed up against the back of the neck, my hand fatigues very quickly.  Unfortunately, I think it's easier to play the notes on the small strings with my thumb behind the neck; it gives my fingers more range and less of a chance of accidentally muting strings I don't want to.   The tips of all my fingers bend back almost 90 degrees, which sucks when you're trying to lay your pinky on a string and it decides it doesn't want to stay straight but would rather bend into a crazy angle.  Great for Fonzi impersonations, though.

 

http://piedtype.files.wordpress.com/2013/03/hitchhikersthumb.jpg

Ummm. And this is simply my take on this. Your grip is wrong. I point the tip of the pick towards the end of my index finger. Which allows for very subtle movement when strumming, muting,, and string skipping. I use my ring and pinky finger as anchor,, usually using the pickup as a reference point. Slide your hand up slightly to palm mute and I can roll into pinch harmonics with very little effort. Just a suggestion. You play well but there is a LOT of extra movement there when you're strumming. You are covering enough space for a full G and are only playing 3 strings. I realize that I do strum wide at parts but that was more of an emphasis thing. 

 

K, first, of all, thanks for the input :)

So, to be clear, you're saying that when i do power chords i strum too wide and i should correct that ? What about "single notes" ? like in the solos ? And if so, how can i correct that ?

On a different argument, i can do palm mute on power chords like on dookie or something punky, but only with down strokes, when i try to alternate picking palm mute power chord the sound is weird, could it be that my hand movement is the reason ?

Well, looking at how you are holding your pick. It would appear that you kind of have to rotate your hand over in order to pick. Seems like that would be somewhat uncomfortable. And to be clear. The tip of my pick basically extends out where my fingertip would be if I were holding my finger straight. It's very relaxed and doesn't require much pressure. This is a perfect example of why I really wish we were in the same room . It would be much easier to explain.

Perhaps this will help explain what I'm saying a bit. The video pretty much sums up what I'm seeing. By your pick grip being as choked up as it appears, It is creating a LOT of extra effort. Which could account for the weird thumb flicking thing you are describing. I'm not an instructor. But I broke both my wrists when I was 17 and figured out how to do things easier. Yes I still played guitar while wearing 2 casts. https://www.youtube.com/watch?v=awiOWXyBTjA

Yeah to expand on these comments a bit, it looks like your whole right arm is very tense...try mindfully relaxing when you're playing. Of course we want to do what's natural but we develop some bad habits. I've had mixed opinions about anchoring...it keeps the hand in one spot, so you have to kind of twist it to pick proper (the way attitude is suggesting). I anchor too though..try just anchoring with the tip of your pinky.  I wouldnt worry about what your thumb is doing, but I would worry about wrist/hand pain, you shouldn't have pain, try practicing relaxing your wrist and arm.

Here's a decent article:

http://tomhess.net/Articles/PrinciplesOfGoodPracticeHygiene.aspx