It also depends on your doctor. This is my 4th pregnancy and my OBGYN has always told me it's okay if I go into labor around the 36th week and they would stop it. The only reason I can think they would have stopped your labor is maybe they think it is a little baby? I would just ask your next visit.
They'll probably stop stopping labor after 37 weeks. It's a great question to ask your doctor!
Riot_Queen made some excellent points. Also, bare in mind that weight estimates can be vastly inaccurate - your twins could weigh considerably less than 6 Ibs.
Because technically you're not full term yet.
Full term is concidered at 38 weeks. Some say 37 but most I've seen is 38 weeks.
And every day counts when it comes to your babies' developement.
I can imagine carrying twins how uncomfortable you are. My daughter was born at 10lbs 13oz and THAT was horrible. I bet you feel even worse.
If you go into labor again after 37 weeks I don't think he'll stop it.
Another problem with L&D with twins is if one or both are not in the right position. It can become very dangerous.
If he doesn't know what position they're in yet, that could be a reason why he won't let your go through with labor just yet.