i've got problem with my thumb. sometimes while playing some riffs (like ladder in master of puppets) it starts to hurt, and i have no idea why. anyone can solve this?

which thumb, plucking or fretting? what kind of pain are you having? i suggest you stop playing that riff immediately until you get to the bottom of this. i gave myself tendonitis in a very similar manner.

the fretting one. its kinda weird type of pain, like the bone was trying to go back or something. if i just stop playing the pain vanishes after a while, but it always comes back when i play that riff

Does it mainly happen when you play riffs down at the first fret position? You have to push a bit harder to fret notes at lower frets. My hand gets tired/hurts when I play in the first fret position on my B string, but its not a strange kind of pain, just "regular pain", if you know what I mean.

If this is the problem you have, then the trick is to keep practicing, but do it slowly to build resistance. But, make sure its not something weird first, because then it could be dangerous, as basstendencies said.

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first make sure your fret hand and thumb are properly positioned. i would then advise practicing the riff at half speed for a bit, and definitely warm up before you play anything technical. if it still hurts to play after that, try and change the fingering to something less painful for you. if none of this works i would advise not playing that riff. i had to stop playing for a good 2-3 months due to tendonitis from a similar situation. best of luck.

and how should i place my thumb? opposite to first finger or little left from it? i guess that just happens when i play riffs in which i need to move my hand slightly... got what i mean?

you want your fret hand thumb smack in the middle between your pinky and index fingers, with the pad of the thumb on the neck of your instrument (kind of like a pivot).this should give you a range of about 5 or 6 frets in any given position,depending on the size of your hands. try looking for lessons on economy of motion, and proper fret hand positioning. not only will you find you experience alot less pain from tough riffs, but it will shock you how much faster and more efficient your fingering will be as a result.

you should be able to reach the aforementioned 5 or 6 frets without lifting your fret hand thumb, but simply by pivoting it. otherwise you need to change positions (i.e. from 1st position to 2nd, etc, etc). i hope this makes sense, its difficult express this without you actually seeing it firsthand.

Here's a perfect example of this, and something that is excellent for exercise:
medium tempo
|-- e e e e e e | e e e e e e | e e e e e e | e e e q- e |
+---------------+-------------+-------------+------------+
|6:-------------|-------------|-------------|------------|
|8:-4-1---------|-4-1---------|-4-1---------|-4-1---7--7-|
+-------0-6-3-0-+-----0-6-3-0-+-----0-6-3-0-+-----0------+

It's a riff from "Staring Through the Eyes of the Dead" by Cannibal Corpse, taken from a tab off of this site. I don't know if the tab is right, but this riff is awesome practice anyway. What you should try to do is play the entire thing without sliding your thumb up or down the neck. Just pivot between the low and high notes, it will kind of make you feel like a violin player (they seem to do that alot).
Be careful, this riff (if I remember correctly) can get tiring real fast though.

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aight, i check things you told me and ill give ya the results