So i was playing as Brice who has 8 points in strength, but suddenly after playing awhile in underwater labyrinth i noticed that now he only have 5 points in strength! What gives???! Is that a bug or what?

Keep track of the weight that you can carry without being over-encumbered. Especially those rocks that are useful for pressure plates are quite heavy. If you carry too much, your character will temporarly lose strength points due to being exhaused, especially if you keep fighting with too much weight on you. You can check your status when you select the scales icon in the inventory screen.

Actually it was fatigue from constant fighting, strange that it is not in status window..(or displayed, but just for 1 second)