Should I message this girl I like?

There's a girl who I met in my drama society who I have a bit of a crush on. We didn't really talk much for the first semester, but since Christmas we started talking a bit more. The first time we properly talked was when I and another guy were walking back from rehearsal together. We'd kind of been casually chatting between ourselves but after the guy went off she started talking to me and I could see she was really trying to engage in conversation with me. Given the circumstances, I never really thought much of it and we didn't talk again for about a month. I followed her on Instagram that night too, although she didn't follow me back.

Then one night when I was coming back from a rehearsal, she was walking alone a distance behind me, and at some point she just walked up beside me and started talking to me, asking how I was and other things. The conversation was really friendly and it went well. It could have been that we were naturally walking at different paces but from my perspective it really seemed like she was trying to catch up with me.

She would always acknowledge me at rehearsals after that, even if just a quick smile. But unfortunately after the play we were doing had finished she stopped coming to the societies for some reason, so I saw much less of her. The last time I saw her we just passed each other in the supermarket, and she was with a friend. She said hi to me and briefly asked how I was, and I responded and then we just carried on with our shopping.

We're friends on Facebook, so seeing recent posts that pop up from her now, it seems she's going or gone home for Summer and so I won't see her again until September. The thing is, I don't particularly want to leave it like it is for that long. I was thinking about messaging her, just to wish her a good Summer and something like that, but I don't know if that would look odd for her if I'm just messaging her like that out of the blue?

I haven't got a clue where this could potentially go or whether it will just stay exactly like it is, but I feel I want to say something rather than leave it for months without word. I just don't know if I'd look odd messaging her out of the blue? Thoughts on what I should do?