I accidently banged my wrists together hard yesterday?!

I accidently banged my wrists together hard yesterday?

the veins on my left wrist came up as thick as a mcdonalds straw.today the bruising is extensive.why did this happen?

1 week ago
i was playing with my dog.

It takes some force to make a serious bruise, though the veins on your wrists may be quite near the surface.
Bruising is a result of blood escaping from an internal wound. Your blood has clotted to stop the bleeding (good) the area around the bruise will be sore for a while.
It is a bit late but get some ice in a bag and hold it to your wrists to try to reduce the swelling a little.

Perhaps you should get a smaller dog to play with?