to talk to him, or not to talk to him?

I've been seeing a boy for a few months (6/7?) exclusively now, we're both at uni and summer got in the way but we kept in touch throughout and were kind of seeing eachother now that we're back. Towards the last few months during summer he started being really flakey and bailing on plans last minute, or not replying till after we were meant to have met up, I didn't make that big a deal of this although it has been a fair few times. I should also mention that his father's been very ill throughout this time and died in the summer.
I was very happy to be there for him, and we did talk about it a bit but as we're both busy I didn't really get to see him that much.
Last year it was more him making the effort, this year he hadn't as much, but we met up a few times, going out for dinner and staying in.
A few weeks ago, his friend had a houseparty for his birthday, they were all really high and he spent the night getting with another girl. I haven't spoken to him since, he didn't tell me but friends did.
I am really hurt, but I realise that we weren't officially together, and I do miss him and recognise he's going through a hard time. I do want to help, because it's hard to stop caring, but I don't want to look and feel stupid.
It doesn't help that she wasn't even some random girl, they've been in a few photos together since. I don't know if anything is happening with them or not. I just am not sure how to act. Do I have a right to be annoyed? I'm just so confused as it all happened so quickly!

As you are not officially together then he can do as he pleases, so therefore you don't really have a right to be annoyed. If you were together then yeah maybe you'd have a right to be miffed.