Sorry, just saw this!!
It does sound like the beginning of the 1st stage. Keep her close by and check on her frequently. I see Breny already answered the question about starting the new thread. Click on Breeder talk and in the top left hand side you will see the button that says New Thread. It is right under the link "User CP". Good luck with your babies
I noticed you said you had heating pads in your whelping box, just make sure there is enough space for the bitch to move away from them if necessary.