As most of the other users who have this problem, it's a hardware issue. Really, after 3 weeks, before you have this problem?
I have the same unit st15i and only a day before reaching 3-weeks. Hoping I won't have this problem.
What color is your unit and model made? (you can check it after you remove the battery) mine is 11w31m
Samsung Galaxy SIII, Xperia Play, Xperia Neo, Xperia Mini, HTC Wildfire