I see your question was quite a while ago but since nobody replied or if you've been getting the run-around from doctors...
Pelvic Floor Prolapse... It is your birth canal part of your vagina that has given out - not your rectum. Before my reconstructive surgery I had to stick my hand up there and push on the rear vaginal wall so the bowel didn't just bulge into there from the other side. Physics dictate that if your vaginal walls are weak a hard poop will bulge out where there is least resistance. Your bowel/rectal/anus area are all lined with muscle so if your vaginal walls are thin and weak the mass will naturally find the easiest exit when you push. Unfortunately the little butt hole is muscular and smaller in comparison so it's that law in physics where things will travel the path that has the least resistance.
Now I have this in my family history but all of us who suffer from this have given birth vaginally at least twice to babies on the larger than average size. If you are trying to conceive and have never given birth before I would research "Ehlers-Danlos Syndrome" if I were you. I have it and it looks like it is a gift that has been passed down for at least 4 generations now. My 16 year old daughter was the one who was diagnosed with this genetic condition and I have been butting heads with her OB doctor over the c-section my daughter and I want her to have so she doesn't end up with pelvic floor prolapse while still a teenager. Her doctor feels it is unnecessary surgery but if she delivers via c-section she may be the first one in my family NOT to end up with a hysterectomy and eventually total pelvic floor repair surgery later on in life.
To have pelvic floor prolapse without ever being pregnant or giving birth vaginally is pretty rare. If you have been able to conceive since you asked this question I'm sure your symptoms are a million times worse. If you haven't conceived yet I can tell you that pregnancy will make them horriffic! I had no problems with my pregnancies or deliveries. My problems were later on in life from them. Now my teenage daughter, on the other hand, is having the pregnancy from hell. If you are also more flexible than most people (or were before it was replaced with osteoarthritis) or have velvety soft stretchy skin it soulds like Ehlers-Danlos syndrome (which the pelvic prolapse is just one of many problems from the disorder) and you should research the criteria and see a genetic counsellor if you feel you meet the criteria. There are other connective tissue disorders that carry similar problems too.