I feel like I'm completely losing control over my emotions.

What I mean is that I've always been a very calm, reasonable and kind
person, nice, open and helpful towards others but recently I just can't
handle my self anymore.
I'm constantly sad and angry for no apparent reason, I became easily
irritable and I started treating people badly even if I don't want to and
even if they didn't do any thing to me.
I have this moments when I get really, really frustrated and it's like my
brain stops working properly, I can't think straight and I can't concentrate.
I also have bad headaches.
This happens a lot while I'm at school and most of the time I miss entire lessons.
But then it's like my brain shuts itself off and everything goes quiet,
and I feel extremely happy and excited. It doesn't last very much though
and then it all happens again.

In my opinion, these are all the emotions that I've held inside during
some difficult periods of my life and now they want to get out.

I'm not sure what do to and I need your advices.

Thanks for reading and wish the best to you all