A.D.
Yes, it could possibly be twins, but it could also be a singleton! Here is the question...when you say she is 6 weeks along...is that from the time you believe she conceived? If yes, in that case, counting from her last period (the way the medical field does to count "weeks", where the first 2 weeks are actually, "pre-pregnancy") she would actually be 8 weeks along. Now, if that's the case, they might be able to see the heartbeat(s) now via an ultrasound (hearing heartbeats on a doppler takes a while longer...)
The fact that she is thin and had a very flat belly makes it more likely, I would think, that she could look like she's showing earlier. There's not much room for hidden expansion in a washboard belly! LOL At 8 weeks (and I was thin but not VERY thin) I was already having to squeeze myself into my jeans, and felt much fuller around the middle than I thought should be normal. Also, could she have been "too thin?"...she may have needed those extra lbs to support any pregnancy and her body got her appetite and metabolism on track early on. A lot of the "bump" this early on, is fat buildup and bloating and water retention (some of us are lucky enough (yeah right) to look pregnant with PMS each month! LOL).
So, don't worry about twins too much yet...but if it is...double the love!