small patch like area in beard - hair grew back white?!

Hi all, please help.
Im 29 year old healthy male, play sports etc but i am quite stressed due to university exams (and general life worries,havent we all?) I have long brown hair on my head no grey hair anywhere and none at all in my full beard either.

HOWEVER, last week out of nowhere,i noticed about 6 grey/white hairs when i shaved, they were all bunched together in a small circular area. Looking at it from a distance when it was shaved/stubble it looked like i had a patch in my beard.  ...but I believe it is NOT alopeica barbae, as there IS hair here, but it is pure white/grey!!!?!

I thought if it was going grey normally one,maybe two strands/stubble scattered around my face would start to go grey,.. but this is a small patch, the size of my baby finger thumbnail just up from my chin, It just appeared over a couple days. Very strange. For example my father is 63 and his hair on his head is only starting to go grey recently.

I dont know what it is. Will the pigmentation grow back? Is it from stress? Is it medically cured?

Please someone help,
Thanking you so much.
Jon