What should I do about a coworker who always stares at me. ?

I catch my female coworker staring at me multiple times a day. Even when I dont look, I can feel her looking at me. I thought it was because I was new and she was maybe keeping an eye on me but 5 months later and she's still doing it. Today she told me the pants I was wearing are my Friday pants because I apparently wear them every Friday. Shes just weirding me out and I'm at the point where I feel like im going to snap and I dont want to do that but it makes me extremely uncomfortable.  I just dont know what to say to her or maybe HR?

(Shes 20 years older than me so I know its not a crush or anything. I do know that shes VERY nosey though so I think that's part of it)