Now, before rambling on i should disclose that i've NEVER had a
relationship with a coworker and always avoided these at all costs.
I have no problem reading girls' intentions in clubs, bars, even
just ordinary day 'game', but i'm very inexperienced with reading
a coworker's signals.
There's this new (very attractive) young girl at the office, who
seems aloof towards me whenever someone is around, but
when we're alone she suddenly changes and becomes friendly,
smiling and talks to me differently to everyone else. With the other
guys and girls she talks with a slightly nervous higher pitched
'girly' voice and quite giggly, with me it's a lower, more
'comfortable' voice tone. I notice these things...
Her behaviour is like chalk and cheese - if anyone else is around
she barely mutters a "hi" and won't even look at me....lol. She's
never like this with other coworkers. I'm especially interested in
girls' opinions on this... does she maybe have feelings for me and
wants to hide this from the other coworkers? (she's quite shy,
younger AND only 3 weeks into the job).

This is unbelievable! Why don't you just face your work before you make silly mistakes!

she definitely likes you. Go for it