Injury? Too bad you didn't include a picture. I have seen similar bruises on pregnant pts who wear tight pants, or lay on something that leaves a mark. If there was no trauma, injury, pain, or cramps - I wouldn't worry about it. Show your ob. It could also be a swollen vein that resembles a bruise or simply a stretch mark that looks different.
Be seen. It is probably nothing, but you should be seen anyway.