Why does my heart feel so bad

Why does my heart feel so bad ? Why does my heart feel so sad ? I haven't slept in a long time i feel, i tried to be cheery as i had promised myself to be. I kept myself busy all day, i smiled through it like a well behaved girl. I smiled at the right moments, i said thank you at every place that was necessary, i helped out in the kitchen and even did the laundry.
But at the end of the day, sitting up in my room, i feel the feelings creeping back to me. Why does my heart feel so bad ?
I made plans for tomorrow, will be spending the day with a friend making jewelry. I should be excited, shouldn't i, but no, i have no excitement or feelings in my heart at all.
Where did my positivity disappear ? Not even shopping these days can win my heart. I try to do brainless things just to humor myself, but no, nothing helps to get the stars back in my eyes.
I have a holiday planned soon, tickets are in hand. I should be excited shouldn't i ? But no, suddenly i feel i am not. yet i am sure to smile trough it, like an artificial doll, and make sure i pretend to have a ball. This is what i feel my life has become, a pretending little plastic doll.
I sway dangerously between emotions like a roller coaster ride and sometimes the swings it takes gives me frights. One minute i am happy, or at least pretending to be, and then the next minute i am down in the dumps, as down as down can be.
I wish this feeling and the heavy heart go away soon, because i really have no reason to feel so blue. Did i not just promise myself a few days back that this is my time to enjoy as much as i can ? Please help me in at least understanding this feeling, because i do not myself, no matter how hard i try.