Fake Friends?

So I'm a sophomore and I go to a private school which means I don't have many friends. About three years ago I met my best friend and she has been there for me through everything within the last three years. Yesterday when school dismissed I was pulling out of the parking lot when I see my 4 best friends (3 of which are guys) hanging out talking. So I see Luke (one of my best friends) pulling out of the parking lot with Jordan in the passenger seat. So I call Josh (her brother whom I also hang out with) and ask what's going on. He said that they were going out to eat and then shopping so I was like okay whatever, ya know? But then I got to thinking and was like wait I had no clue about this all day like why didn't they invite me but I didn't want to sound petty so I just didn't say anything. Today at school Jordan pretty much avoided me all day because she was hanging out with Matthew (the other guy in the group). So I ask her what was up and she just said that they hung out yesterday and told me about her sitting in his lap and they kissed or whatever and I was like oh okay cool. She talked about how they had such a great time and I just replied with, "Thanks for the invite." She said that she "honestly" didn't think I'd want to come (even though every time I go somewhere I always invite them). I think she just wanted the attention being the only girl but it just stings that she didn't at least call me about it. Am I being petty or overdramatic? I just don't know what to think about it. I felt like I was invisible to them today because they only talked about the fun they had yesterday and it honestly bothers me but I wouldn't dare tell them that. I feel like I'm being stupid about it. I don't know. Help?