I don't like that my boyfriend did this?

So my boyfriend has/gets a lot of tattoos, and a while back when we first started dating he allowed his female best friend to come to one of his appointments. (This is the same female friend from my other posts). I had never been to one before, but I really had wanted to go. I wasn't able to because of school. It really bothered me that he asked her to come with, especially since that appointment was the very start of a tattoo. It felt like since I couldn't go, she was the next best thing, as though she was replacing me. (they also went to get a piercing of hers after his appointment). I also felt like that's the kind of thing that he and I should do together as a couple, not her and him. I felt left out. He didn't see it as a big deal to have her come with, but I expressed how I felt and he understood after a while, and we were all good.
But now, I find out via Twitter (kind of stupid I know) that he ASKED her to come to his next appointment. And it looks like it's going to happen since she's free.
I feel somewhat betrayed. He knows very well I can't stand her (for many reasons) and that I feel extremely bothered that she's going with him to it, yet he goes and ASKS her to come...
How do I go about handling this? I don't want to seem stalker-ish and clingy, and tell him I saw their conversation.