how many wet diapers is she having? If she has at least 6 per day, she's getting enough.
There could be a couple of issues, she might have reflux (does she arch her back, spit up, appear to be in pain?) you might have an oversupply (does she ever choke or gag while nursing?) It doesn't sound like you have a low supply because I think she'd be nursing more, not less, if you did. Has she always done this or it is a new thing?