Problems with D7000 exposure and metering with Aperture Priority please help.

I took alot of photos yesterday.  And on one spot the exposure was all over the place.  I put the D7000 on Aperture priority.  Sometimes the aperture was at 2.2 and sometimes at 2.8.  Same scene.  When I went from 2.2 to 2.8 the camera actually used faster shutter speeds.  Underexposing the photos then.  Why would it do this.  Shouldn't it have done the opposite?