I [usually] feel OK when I'm out and about, but once I'm at home I break down into tears... what is wrong?!?

I'm feeling really confused and afflicted, when i'm out i tend to feel okay most of the time, but when i'm home it's rare that i'm even in a mediocre mood, i am straight up miserable and spend my day in bed crying over this sadness that i cannot pinpoint. my mom is concerned but at the same time i can sense that she's getting SICK of me being sad, hell, I'M sick of me being sad, but it kills me that my pain is washing onto her... I'm at my f*cking wits ends, please give me advice as to what may be wrong with me :(