I Have Been Running For Years With No Problems. I

I have been running for years with no problems. I am a 40 year old female and suddenly the past couple months after I walk up a set of stairs my legs are tied and sore like I've just ran a 1/2 marathon. It's not so much painful but tight, tired and annoying. I notice they feel heavy when I begin my runs lately too but the stairs thing has me puzzled because I've never had issues climbing stairs and don't understand what it could be. Any ideas?