I attended a neuroscience conference a few years ago and an investigator presented data on the effects of multiple headers and didn't report any significant effect. However I refer you to this website http://www.neuroskills.com/pr-headgear.shtml

lots of kids and young girls in particular play football in the US and protective parents want their children to wear protective headgear which is on the market. it doesn't help at all.

I suspect that the bruise you got was more a result of the hard slapping effect of the ball as it deformed around your forehead causing blood vessels to rupture below the skin rather than purely a hard surface contact