I Would Like To Know Why I Have Such Difficulty

I would like to know why I have such difficulty in climbing my basement stairs up. My thighs feel like they way an extra 100 lbs each. At work I can climb our front steps at an angle and it can be tiresome but coming straight up is very difficult and I get out of breath and takes longer. I'm 62 and take a beta blocker for irregular heartbeat (benign pvcs).