I hope I am understanding this right. So forgive me in advance if I got it all wrong. When they first did the speculum exam, were they examining you to see if you needed a cervical cerclage (sp?)? Basically a stitch in the cervix to keep it closed? Is that what you meant by them deciding not to stitch? They may not have stitched you at the time because you were cramping and contracting, which can indicate labor is coming. Did you recieve any labor stopping medicines? Like Magnesium sulfate? These types of meds are effective in stopping early labor and they can cause the cervix to close back up, especially if the cervix is only dilated because of cervical incompetence (which is often why they stitch a cervix up). You very well could have had slight dilation upon your first exam but after lying down for that 24 hours, the pressure would have been taken off of your cervix, thus allowing it to close again. That can happen naturally and also medically. If you have an incompetent cervix, any direct pressure will allow it to open, and laying down will relieve the pressure, thus it closes again.
For you, premature labor "could" happen, but doctors these days are VERY good at getting women (even women like myself, who have had premature rupture of membranes) to at least the viability stage of pregnancy. Sometimes even to full term. Doctors can give labor stopping medicines like mag sulfate, they can give steroids to help speed up the lung development of the fetus, they can also do artificial amniotic fluid replacement. (Basically sticking a catheter into your uterus that delivers sterile solution around the fetus that acts as amniotic fluid does, used to prevent the umbilical cord from being compressed) Right now, honestly speaking, if you were to go into labor, not much could be done, but miracles can happen as well. The best you can do for yourself and your baby is lots of bed rest, minimal up and down movement and plenty of good nutrition and water. If at your 22 week exam, the doctors find more troubling signs that labor is coming, there are many things they can do to keep your baby inside of you for as long as possible. Best of luck to you.