Yes, it can be. Sometimes he can bang into your cervix, and cause some bruising and bleeding that can take a bit of time to come out. After or before your period, their can also be some menstrual debris in your uterus. This can come out out when your cervixrepare for ovulation or menstruation, when you have sex or masturbate.
It is also possible that the small micro tears that develop in your vagina during sex, bleed.
Also monitor your discharge for a yeast infection.
Like always, keep well lubricated during any sexual activity. Just because you are wet when you start, does not mean you will stay wet the whole time. Stay safe, and see your doctor if you have more worries.