Could this simply be her "Heat cycle"? Dogs do not go through menopause. If they remain "entire" they will have Heat cycles as long as they live. It depends how much blood she is losing, and also her general well-being.
I she is showing any signs at all of being under-par, or unwell, this could be something has gone wrong.This blood could be coming from her womb, or from her bladder. Either way she does need to see the vet as soon as possible.
When you say she had tumors removed....where from? Her bladder, womb, or somewhere else?
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