I know I burned the LEDs because when I tested after with the diode testing feature of the voltmeter, they didn't work anymore ; also with the 3.3V voltage they didn't work anymore.
Strangely enough, one of them worked, while the others burned out.
The LED in the LED matrix don't look light normal LED, they look like "flat LEDs". Is it normal or is it the sign of "low voltage matrix", what current should I put through the LED then and how can I know if the matrix is of this type?