More info would help - how much blood, is it just dripping out or sprayed to the sides?
Muscle, liver, artery, kidney or heart blood is usually dark or solid red.
Lungs, windpipe or mouth, right along the diaphragm, or low through front part of back legs often pink or light red.
Gut may be very dark if there is intestinal material in it, or may be lighter if through the first stomach.
If there was any hair at the site of the hit or on the arrow, that could also be an indicator - long, dark hair is usually higner on the body, long white hair from the belly or inside of thighs. Shorter hair usually from neck or legs.
Since you think you hit it back, my guess would be stomach or front part of back legs and low through the abdomin.