S.K.
I have to tell you that it is extremely likely that you are pregnant. I am a L&D RN and have been in this field for 7.5 years now. You can get pregnant the 1st time you have sex after delivery, even if it's only a week or two later! Breastfeeding is not at all a contraceptive so I would repeat your pregnancy test and if it turns up negative again, I would schedule an appt with your OB/GYN to have a blood test. The fluttering feeling in your stomach could easily be a baby - most women feel kicks closer to 16 weeks with their 2nd child as they now know what the feeling is like and tend to notice it faster than a 1st time mom does. You could easily be 16 weeks pregnant now since you delivered Dec. 1st - it depends on how quickly you had sex after delivery.
To everyone: If you are not interested in getting pregnant quickly after delivery, there are many options your doctor can offer you while you are still in the hospital on the postpartum unit. We often give shots of Depo-Provera so that women are covered with birth control for the 1st three months after delivery. This way when you have your follow-up at 6 weeks, you won't be pregnant already.
I have delivered many patients who have babies 11 months apart, so don't think this isn't possible. Talk to any L&D nurse and they'll agree - you need to use protection from day 1 after delivery if you're not looking for another baby in the next year!
Best wishes - either way, it's good for you to know so if you're pregnant you can continue to take good care of yourself to have another healthy baby.