It's common for third grade to be the age-period when academic weaknesses really start to show up. If she had a true learning disability you would probably have heard something before this. Maybe you can see excatly what shes has trouble with. Usually it is one or a combination of reading, math and writing. Cursive sometimes causes problems and kids stop wanting to do work if they can't write well. (I see an another post on this topic).
Since she doesn't seem troubled by it, I would give all the support I can, talk to the teacher more, and remember that everyone isn't a great student--th