Do you think its right for me to be annoyed at my colleague?

Here's the situation. I work part-time for this company, so I'm only on the premise once or twice a week. I get along brilliantly with everybody, including the person in my question, but her attitude towards me changes everytime a certain member of staff...

(who I've also become mates with) is present. I'm pretty sure she's got a thing for him, and I don't really care, but she just acts very dismissive and snooty towards me whenever he's around. As soon as we all finish work for the day, she practically shoves me out of the door so she can be alone with him, even though they both work full-time at the place and get to see eachother often. I get along well with both of them individually, but whenever its just the 3 of us, I feel very uncomfortable.
Do you think I sound jealous, or that I'm getting irritable for no good reason?
How would you respond in my situation?

It sounds like she might feel a bit insecure about you (especially if you are a woman) around this man. Try winning her trust so she understands that you have no interest in her crush (ex. talk about how much you love your boyfriend or single life). Also don't take it so personally because it has nothing to do with you since it's just her insecurities. Since you only work part-time you aren't a real part of the group (day to day getting to know eachother, building trust, etc) and you could be perceived as an exterior threat. Until she understands you aren't, she'll probably continue her ways. Be patient and don't get too rilled up over such a small detail!