A lot of times it is caused by the rise in BP during intense, heavy lifting. Breathing does help. Blut Wump is right, get your bp checked just to make sure it isn't high to begin with.
One time after a heavy set of deadlifts, both my shins were bleeding, my nose was bleeding, and my hearing was impaired for about 30 seconds. i thought it was pretty damn cool. But in all seriousness, I agree to hydrate, breathe, and get a routine physical to rule out any non-training related issues.