This is actually the situation I found myself in a couple different times so I can not tell you for sure what I think that it is. I would say the best course of action is to NOT rely soley on the ultrasound.
You want to make sure they are doing a quantitative beta hcg blood test. Since you are still BF and you have NO idea as to when you ovulated you dont know what gestational size they are looking for. This almost caused me to have a D&C of the pregnancy that resulted in my precious little man Wyatt.
The Beta will show if your levels are going up or down. If they are going up then all is well and they can do an ultrasound once your HCG is over 2000 they will be able to see more than the sac. If they are going down they can monitor it.
Since a sac was seen in your uterus there is no real need to worry about ectopic pregnancy but it could also be a molar or partial molar pregnancy. I am not trying to offer any scary news but I am trying to be realisitc. Since I have had so many miscarraiges for different reasons I know most of the possible outcomes.
Recently I was in the same perdicament for me it turned out that I miscarried. I am hoping for you that the first few days were actually implatation bleeding. which would be perfect because then there wouldnt be much for them to see
