It could be anxiety..and maybe I'm a bit cynical atm..but this confusing behavior. Fine in groups..awkward alone..awkwardwkward when a coworker of hers shows up unexpectedly when you are alone..no phone number only contact thru messenger???....I'm getting redflags here..I think she is hiding something maybe...maybe it's just that she's still kinda back and forth with the ex?
Or it could be there are broken up but he still has a bit of a hold on her?..idk...whatever the case..if you are feeling confused...like prefab said...I would ask directly, but not aggressively. If her response is vague or she is uncomfortable with responding...just give yourself some space..wait for her to contact you. Hopefully she will be honest about whatever her deal is...but even if she is just shy with trust issues..you gotta let her know that her confusing behavior is messing with you too. It's not all about her comfortability, you shouldn't need to be confused and uncomfortable either. It's a two way street.