Urine was slightly red after masturbating. too hard on my penis?

Probably: It takes only a microscopic drop of blood to turn urine pink. If you had an aggressive session of masturbation, you very well could have damaged a area of the urethra or opening at the tip of the penis and thus have slightly red urine. I would be cautious, stop for a time, evaluate and if any concern, get checked by pcp or urologist. Re-evaluate your activities, techniques and practices.

Be more gentle: Probably cause your uretra ot bleed a little. Be a little less rough.