How do I fix stain after sanding drips in polyurethane?
Many finish coats, including polyurethanes, can slightly darken a finish when applied. Your sanding, if it did not cut into the stained surface, may have just removed that darkening poly layer.

You should try applying a coat just to the area you sanded, feathering the edges with a nearly dry brush. Let it dry and see if the color has evened out.

If not, I would recommend sanding the whole side. If it has lightened significantly, re-stain the side and then re-poly. If not, just re-poly.

You may want to look at some of the wipe-on finishes. These are satin finish (not high gloss) and go on easily with much less chance of runs.