Hi. It could probably be minor injury that has resulted in a small amount of blood, if you are not on any medications and have not consumed anything that has a orange colour, dye or hue to it; then the cause is most probably the sexual intercourse that has caused minor injury. Just make sure you avoid penetration for a while, and you should be fine, I do not see any major cause for concern. And in the future, avoid repeating this by informing your partner. Best wishes.