i think your normals might be facing the wrong way, select the faces, and in the normals menue select reverse normals from the drop down menu, i'm not sure what you used to bake out the normal map, but if you did it in a third party softwaye like xnormal, topogun or even zbrush then you will need to export out the low poly model out again (having reversed the normals) and rebake the normal map again,

hope this helps