I Am A 62 Year Old Woman. I Have Severe

I am a 62 year old woman. I have severe knee pain, one knee only, inside part of knee and down toward my foot maybe 6". When it started I noticed I had red splotches on the hurting part of the knee that went away after about 4 days. It has been hurting about 6 weeks. Only the one leg. I sort of thought maybe it was shingles but now I'm doubting that. Anyway I wonder what it is.