viral spider icon?

Hey,

Yesterday suddenly an icon -looking something like this (my drawing attempt)- popped up in my dock, the screen started shaking, I couldnt do nothing for the minutes I waited. Because I was working on important data on the usb, and I wanted to save the damage The only thing I could think of was getting my usb and pressing the power button for a few seconds. Then I got to restart my macbook and there were no signs of the icon or the problems.

What could this be?

Thanks in advance,