What to do if you're not invited to party - awkward situation?

[Izzy]

Ok so basically these two girls are having a house party and all my friends have been invited apart from me. The event was set up on Facebook & so everyone was invited thorough that; I don't have Facebook so I wasn't invited.
However, I was really good friends with these girls last year and the year before as we had lessons together, now we only have one lesson but I still do talk to them. I'm also friends with most people in their closer friendship group.
So when my friends told me about the party being organised on FB, I just thought: oh, they'll invite me web I have a lesso with them next because I don't have Facebook. But they didn't say anything to me.
So last week my friend messaged both the girls to ask if I could come to the party. Well their reply: No, there's too many people going already.
I really wanted to go to this party because most people are going that I like and so are all my friends. I thought I was friends with these girls, but obviously were not as close as I thought. I'm really upset because it sounds like a really good party.
(Btw they're not at all bitchy, so I'm quite confused)

I have a lesson with them next week, and I'm worried its going to be really really awkward. What do I do?!?!

(Sorry for going on)