getting her/his weight and length from head to tail will be very beneficial in determining if she is over weight or not.
From the picture, she doesn't look fat at all. The spread out body is normal for bearded dragons, when lying flat, specially if feeling a bit nervous, they do tend to look very wide.
Fat doesn't typically gather around the body like it does with humans. Fat deposits on bearded dragons tend to be on the head (just behind the eyes), base of the tail, the arm pits and sometimes the beard area. More pictures of your dragon from all sides would be beneficial as well.
Checking to make sure she is actually a female, would be a good start. Place her on a flat surface, like a table or your hand, facing away from you. Lift the tail up to about 90 degrees (be careful not to force it up, you don't want to injury the tail). While twisting the tail slightly side to side, look at the area on the tail side of the vent. If you see a single central bulge, it is a female, if you see a dimple in the middle or two bulges on the sides, it is a male. Doing a google search on 'sexing bearded dragons' will bring up all kinds of sites that will have pictures.
Females can lay infertile eggs and many will even if they never saw a male, much less been housed with one.
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PHLdyPayne