What Causes Reduction In BP Levels After Exercise?

When I exercise my blood pressure (taken with my husband s wrist BP monitor) goes down.My usual blood pressure is about 125/65.The last few days I checked my blood pressure right after walking for 30 minutes because I wondered why I feel so weak after walking and my blood pressure was 80/55. I thought blood pressure went up after exercise.This weak feeling is not new, I just wondered if my blood pressure could have something to do with how I felt during walks up little hills and such.