Dog Tail Wound - Seriousness?

Last night I discovered a wound on my dog that I have no idea how it got there. My girlfriend and I noticed some blood spotting on the bed last week and had looked over our pets to no avail. Now last night there was a little more spotting on the bed and my dog was laying near it. My girlfriend pointed out some hair on her tail looked off and upon further inspection I discovered the wound. It was only slightly bleeding, but looked more just exposed that anything. I'm unsure if the spotting from last week was a result of this wound (I assume so) but I'd have to guess the wound had started to heal over and she must've somehow opened it back up yesterday. I have an appointment with our vet tomorrow morning but as you all know pets are our family and I still fret worrying about it. Thoughts on the wound?