Hello. What color is the blood? Is it bright red or dark? That helps us understand where this might be happening. If you have something like a fissure or in general, things close to your anus is going to be brighter red and things higher up will be darker. Here's a whole list of possible causes. https://www.mayoclinic.org/symptoms/rectal-bleeding/basics/causes/sym-20050740 It can be very simple and it most likely is. But you DO need to see a doctor asap to determine this.
My son had the same issues, while taking creatine (to get muscular!). I googled the 2 together and there are many young guys complaining about this. I know, they say is safe:-( Are you taking any?