Urine was slightly red after masturbating. any damage done?

Doubt, but observe..: For possible urine infection or local irritation. In females urethral irritation is easier to take place in this case as opposed to males. Also, have a level of suspicion for possibly unrelated causes e.g. Ate beet earlier or have a different reason for blood in urine like a stone disease where the color change took place as a coincidence.

Probably not: You were probably a little too vigorous and caused a capillary bleed in your urethra. This should clear up quickly.