cant always be happy?

Yesterday I was sad, and almost broke down infront of everyone, but I didnt. I dont even know why I was sad, I just had this really sad feeling inside of me, and tears came out. I almost broke down infront of AJ, but I didnt, I didnt want to cause a scene, so when he told me to go wash my face, I went to the toilet and broke down. Its stupid, I had nothing to be sad over. I think its probably because I'm just always happy, and you just have to be sad sometimes. During these past couple of months, I have been quite happy, had nothing to get sad over. I get sad when we argue, but it's not like I'm sad for like days or weeks or months cos of it. I have truely been happy lately. Except for when I wass fighting with "her" I was sad over that, cried over that for a while. But besides that everything was going well, it was all okay. I also get annoyed a lot of the time, but doesnt mean i'm angry, I still have that happiness inside of me. Today, I am happy again, but there is a little sadness in me, I dont even know what it is! So uncool.