As far as I know, actual blood coming from nipples is not normal.
I had a female dog who was entire for a while (before I had her spayed) She would get some nipple discharge, and sometimes it was pink-ish, but that was just after her heat cycle. It was caused by fluctuating hormone levels, and usually went along with slight 'false pregnancy' symptoms (also caused by hormonal imbalance.)
After she was spayed it stopped happening.
How old was your dog when she was spayed? At what time in her heat cycle was she spayed?
It is possible, if your girl has only just been spayed, her hormones are changing very rapidly right now, and that might be causing some nipple discharge.
But blood? I don't think this is right at all. My advice is please take her back to the vet to get this checked out.