Vaginal prolapse is fairly common in bullmastiffs, or at least, bullmastiffs are one of the breeds that is most commonly affected by this. It usually presents right before or right after the onset of the heat cycle. Since you have an appointment at the vet's tomorrow there is no real need to worry, however the uterus can prolapse quite far out, enough so as to look alarming. As long as she is still able to urinate there is no real emergency, especially since you already have the appointment.
Depending on how severe the prolapse is, sometimes a catheter is needed if the dog cannot urinate. If the vet feels that the condition can be managed at home, you will need to wash the area every day with a saline wash. Sometimes antihemorrhoidal cremes are applied to the area to shrink the prolapsed tissue. In some cases it is serious enough as to warrant surgical intervention to have the hyperplastic tissue removed. Your vet will determine which course of treatment is necessary after he sees her. Please let us know how she does at the vet's.
Ghilly
No I don't have experience of this, but from what I have heard about it, the best option I think would be to take her to the vet tomorrow (don't wait any longer than that) and get this properly checked out. It could well be that her best option would be to be spayed. And she may need antibiotics too.
As she seems very well in herself right now, this is not something for you to get really scared about. But at the same time, prolapsed uterus is a serious condition, and needs attention as soon as possible.
no dont get her fixed! no way, my pitbul had the same all the vets did was push it back in a lil and put a stitch through to hold it. 2 years later my dog is fine, it mite show a lil after second heat but it gets better. my dog never showed again but you mite wanna get advise if you want to bread her, problems could occur.