Well, first of all, u/s is generally not all that reliable for determining sex of a baby prior to 18-21 wks or pregnancy. The tech may be able to take a pretty good guess, but it won't likely be as reliable as it would be later. You'll just have to see what kind of image you get & how the baby is positioned.
Secondly, this doesn't sound horribly abnormal to me. I guess there's a possibility that you could be pregnant w/ twins, but it could just also be that this is your 3rd pregnancy (I think you said) and that with each pregnancy, your round ligaments get stretched & weakened a bit more & your body has already been through pregnancy & "knows the reutine" so it starts expanding "sooner". It may also possible that you're further along than you originally thought.
I really wouldn't worry about it too much. I'm sure that everything is fine. If having an u/s earlier rather than later would make you feel better about things, than that's fine. But in the meantime, just try to relax, trust your body & trust your baby. Everything will be fine!
Good luck!