How old are your bunnies? Mounting is often done by both boy and girl bunnies so that’s not an indicator of the sex.

Humping is not a reliable indicator for determining the sex of a rabbit. Females will hump as well. Did you confirm that he is a boy? Does he have visible testicles? I’m assuming he’s old enough to determine his sex by looking at his parts too.

That being said, it only takes a few seconds for a male to impregnate a female. Heck, they don’t even have to be in the same cage. Buns have been known to do it through the bars!

So now your female has been exhibiting a change in behavior. There are 3 possible scenarios here:

1. The new rabbit is a boy and he has gotten her pregnant.
2. The new rabbit is a boy and he has not gotten her pregnant. Instead she is experiencing a false pregnancy (a biological response to being mounted).
3. The new rabbit is a girl and, therefore, nobody has gotten anyone pregnant. The behavior change could be a false pregnancy or simply her hormones acting up because a new rabbit was put into her territory.

How long ago did this incident occur? You’ll have to wait 30 days to see if there are babies (if the new rabbit is a boy).

For future reference, it isn’t a good idea to stick two unaltered rabbits together. Sometimes two rabbits of the same sex who are not fixed can get along OK. But more often than not, hormones will cause them to fight… particularly if they are in a confined space. The best bet for having two rabbits with a lasting bond is to have them spayed and/or neutered first. Then you gradually introduce them to each other. And, even then, it all comes down to personality (i.e. they don’t always accept one another).