'Straight' girl falling for 'straight' colleage

Hi there,

Basically could do with some opinions on the little predicament i'm in right now...

I've recently begun to feel attracted to a girl that I work with. I've been working at the company for about 8 months and have known her all this time, but only recently begun to get closer as we've been working together on a project. We've got a good friendship going - not close by all means, but jokey and we chat occasionally. We're not best buds, but in working terms then we're most definitely friends. She's always given me a bit of a vibe, but she's been seeing this guy for about 5 years and they seem pretty serious. It's quite confusing. She's quite touchy feely, but not ridiculously so, just a little bit eye-brow raising but could possibly still come under the straight girl friend bracket (incidentally, some of the thread i've read about a girls spooning and feeling each other up and STILL not being sure if a girl's feelings - what planet are you from? MOST DEFINITELY call that girl out - what is she playing at?!! If this girl was doing that to me then my god, we'd have a bit of a situation on our hands!). 

I find she often looks at me oddly - gazes a bit longer than necessary, generally sits really close to me when we're together, makes comments about people probably thinking i'm hot etc.

Basically, nothing can really happen because i'm in a relationship with a guy I love very much and it would just be so BIZARRE if anything happened between me and this girl publicly - I guess I just fancy her quite a bit and quite enjoy fantasizing. I just wanted to gauge whether she might have a hidden agenda or whether i'm being hopeful? Perhaps i'll just carry on flirting a bit, enjoying a bit of a chase in the office. For a 'straight' girl she pings quite hard.