I wouldn't panic over the comment, but I'd continue to pay attention, as well as continue to have the types of conversations where you "drop pebbles" to see if any info bubbles up to the surface - ie, you initiate the conversation in a very slight way, ask a leading question, something simple, see if it leads to a deeper conversation where you get more info on how she's feeling or what's up.
I wouldn't just let it drop to the background altogether though. My ds was in a semi-similar situation in elementary school. He started out with two fast-friends in kindergarten, but as he moved on in school those two friends kept making other new friends and ds never did. When he was in 4th grade he started wondering why he didn't seem to be making friends like the other kids were, and he told me he was hanging out by himself on the playground at recess. It wasn't too long after that before he announced to us (quite unexpectedly, to be honest) that he was bored to tears with his school academics. We transferred him to another school after we found out about the boredom, and viola, once we had him placed in a better academic fit, he made new friends easily.
Had we described him socially (at school) back in early elementary we (and his teachers) would have said he was a quiet, introverted boy who had difficulty making new friends. At home he was very outgoing and happily played with his sisters. In preschool he'd been relatively social. The reality is - he actually *IS* extremely extroverted and social and he makes friends easily - the not being with anybody on the playground in elementary school wasn't *him*, and was instead a sign that something in his world at school wasn't a good fit.
Best wishes,
polarbear
