Plus one for examining the inside of the tyre where the bump is. I found - happened sequentially (ie front then rear, 1000km apart) on two Giant PS2-L 28c tyres, unhappily one half way to JoG - that both had failed in the same way: the threads had split apart. Clear from the inside. External symptom was side bulge ie 5mm+ and thinning of the tread at that point, I surmise caused by squirming and therefore increased lateral friction, at every tyre revolution.