Girls who stare at you - awkward

I think I posted this anonymously but it's my first time using it so if it doesn't work then no biggie.

Ok I'm not going to be one of those people that go 'Do girls stare at me because I'm so good looking?' since I doubt that's the case. Whether I'm on the bus, on the train or at uni, every now and then girls who I don't know personally stare at me. This makes me feel kind of awkward and shy. I am a shy and reserved guy for the most part. If I ever get any attention like when doing a group presentation etc I get really nervous or flustered. But I wouldn't say I'm a stun silent anti-social person since within my group of friends I talk a lot.

What are the possible reasons girls stare at you? Aside from the obvious one, I think I am average looking, not ugly but no Zayn Malik either. And how should I act in those situations since I probably look like a goofball. And the stares are not friendly either. It's just a blank stare where I can't tell what they're thinking. Maybe I'm just too self-conscious? I mean it's not that big of a problem I guess but I do feel awkward whenever it happens.