What Causes Weakness And Cramping While Trying To Climb Stairs?

hi I am a 44 y/old woman with a 24 year history of asthma. I cannot take inhaled steroids due to side effects. In the past year and a half when I climb stairs or hills I get weakness and cramping in my legs and they buckle causing me to fall to my knees. what is this and what can I do about it?