I agree, your concerns are totally valid and they should have had you in for a fetal movement test (can't remember the name).

I had a similar issue with DD. I went to the Ob as a 'ring in' on the Friday with a leak, they told me it wasn't, I believed them, went back on the Tuesday, they said I wasn't, and they expected me to go full term if not over...my waters broke that night...3 weeks early.
Afterwards they admitted it was probably a weakening of the sack or a small tear that was leaking, and then it just caved.

You should totally wear you b*tch dacks tomorrow and let 'em have it IMO