I'm not sure to be honest.

If you havent had trouble sitting down it may not be broken. It may be that its just healing.

And you did say you had a vac delivery so if the baby was still quite high the whole process may have happened to quick. Meaning that rather than baby gradually pushing the sacrum and tailbone out of the way, it was forced out of the way by baby being pulled out if that makes sense.

I'd still mention it to your Gynae next week if its still giving you grief.