My Fiancé Is 2 Weeks Post Operative For Bicuspid Valve

my fiancé is 2 weeks post operative for bicuspid valve replacement and ascending aortic aneurysm repair. he started having pain in his right side lower back today. Is this common or something to be concerned with? His blood pressure also dipped today. He has been walking 12 mins x 4 a day per direction. Does this have anything to do with it maybe?