Feeling horrible

Today i'm feeling terrible about myself, but i do  not know the exact reason. I'm feeling so depressed and sad, I just want to take the rest of the day off and go home. I'm texted my BF about it and he was busy today at work and he could not reply me. This is making me feel even more sad that I do not have anyone with me when i actually need someone. I'm feeling like crying. But i have absolutely no idea whey i'm sad.