Best advice. Relax!!! If there is no placental tear the baby should be fine. He or she is getting all the oxygen and nutrients it needs (although you should have weekly ultrasounds until delivery to keep tabs on this). My regular obgyns were puzzled as well. It was only after a specialist came in that they realize my uterine muscles tore. Torn muscles don't show on an ultrasound. I was concerned when it happend with my last but it wasn't as if we didn't know what caused it. Because we were aware of the cause, it was much easier to stop contractions. I didn't even need meds to stop the contactions. I just needed to lay down and let my body heal a little.
Honestly, it sounds as if that is what happend to you. After delivery (hopefully around six weeks from now) you should be checked out. If your six week check-up shows that your uterus is tilted, more than likely that is what happend. Unfortunately your muscles will probably not heal good as new (your muscles should heal now with an enlarged, pregnant uterus but after delivery the muscles will have already adjusted to compensate a larger than normal organ so once you deliver and your uterus goes to pre-pregnancy size the muscles will be distended and will most likely cause the uterus to not sit correctly) and your uterus will not be in the same position as most women. Don't worry. It doesn't affect your ability to concieve in the future and as long as you take it easy you should make it to a safe gestational age with this pregnancy.
Questions to ask at next appointment which should be this week or early next:
1) what are you measuring at (pubic line to top of uterus)? Key determination to torn muscles. A fallen uterus is a good indiction that muscular tear has happend. They should measure you sitting up as well as laying flat on your back. Sitting up is more accurate but laying back is easier to measure plus it allows the uterus to shift up which can give a false measurement.
2) does the baby have adequate amniotic fluid? Sometimes torn muscles are enough to tear the amniotic sack. You may have heavy to very light discharge. There is a very simple "dip stick" test to check for amniotic fluid leakage. This should be done at every prenatal check-up from here on out (my doctor was kind enough to give me a few extra strips so I could check at home if I ever thought there was more than normal flow). They can explain how the strips work. If you do begin t leak fluid the baby should be delivered within 24 hours due to infection that could set in.
3) do not be afraid to ask for an early induction. A baby born 3 weeks early is far better off than a baby who has suffered oxygen or nutrient deficiency. I would definately consider an early induction if they are clueless as to why the bleeding began. 36 weeks is a safe range to consider early delivery. If you can get adaquate bedrest for longer, consider a 37 or 38 week delivery. Your doctor should be more than understanding to your request of an early delivery, especially if they have no idea why the bleeding occurred. Although an early delivery may be a bit more painful (sorry, inductions are more painful than natural delivery), your doctor can monitor the baby's vitals from the very onset of labor. If a problem arises, a c-section can be performed in minutes which would be far more safer than you going into labor at home and having something happen. I'm not trying to scare you but I would definately push for an induction. Minutes count when it comes to an unborn child and if you go in to labor at home and something goes wrong, you just lost all those minutes it takes just to get to the hospital. Let alone until they wait another five or ten minutes to figure out you need a c-section... Five minutes ago. If you are at the hospital from the get go, odds are in the baby's favor that he/she can be safely delivered in an acceptable time frame.