What Causes Pain In Back Along With Leg Spasms?

I am 46 and have been lifting weights for over one year. I squat 300 pounds. Every time I workout my legs they become fatigued and want to buckle. This always goes away by the next day. Three weeks ago I went skiing. Just before leaving my back started hurt and my leg would spasm. After sking two straight days with a bad back my legs were extremely sore and fatigued. It has been three weeks and my legs still have not recovered. My knees buckle like I have just finished a heavy leg workout. I tried to work through my leg problem and could not squat 135 pounds. Im freaked out and worried I have ms. Please advise