Search Results: avoiding me after sex

It was my friends 18th a few days ago and we went out clubbing yesterday with the squad. The girl whose birthday it was has been my closest friend since kingergarten but we have a very superficial relationship in that we rarely talk about deep and meaningful things. For the sake of anonimity I'll call her Kate.
Now last year me and this boy who I've been so close with for years disovered we had feelings for eachother and he told me he loved me. We will call him Joe. We had a complicated relationship for months. He was a virgin and wasn't ready to have sex and that was fine with me but I felt at that time in my life I wasn't ready to take the plunge because I thought I would hurt and disappoint him. We ended things and went back to being best friends.
So, Kate has been a bad friend for a long time, doing countless shitty little things but our families are so close that she's more of a sister (thus I cannot avoid her). Joe has been doing bad things since they've been hanging out more.
Last night Joe asked me for a hug and really held onto me and he has social anxiety so I thought it was the usual "I'm having a panic attack help" kind of hug he gives me. Then he told me that him and Kate are now together. Turns out everyone knew and they have been for about a month but flirting and what not for longer.
I see Kate every day and not once did she think to tell me.

I'm not a person that is overly emotional but when I heard this it felt like someone had punched me in the stomach, I went and had a cry and a panic attack after. She's so manipulative and fake (always has been) and I'm so scared he will get hurt. She fakes interests that he has despite me knowing that she hates them. (hardcore music, videogames, etc...)

Am I justified in being angry with them for not telling me? I just feel really disrespected and betrayed. Is it too much to ask that I be made aware of this, even casually? What do I do?

Sorry for the sook guys, I just feel quite down. I need some advice.