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A prolapse of a vaginal wall could have been easily diagnosed by your Doctor,so it must be something different like a fibroid/polyp of the endocervix filing your vagina.
And it must be having a pedicle so you must be feeling it protruding outside.
That must be why your Doctor's asked an ultrasound to see the exact origin.
Also a polyp/fibroid can grow fast but you would have had some menstrual complaints.
So it must be all the more confusing for your Doctor.
It would be fair enough to go in before your scheduled Ultrasound to take an opinion about the nature of this swelling as it has been rapid in growth.
To answer your questions about fibroids,they are benign tumors of the uterus and known to grow almost anywhere in the vicinity of the uterus.Usually they cause symptoms like excess bleeding or pressure symptoms.
I hope I am clear.Please be free to ask any questions.
It could be a cyst too but cannot be possibly caused by a examination.
Yes it can be an inclusion cyst caused by TRAUMA but rarely by an examination.