Saying No

Takde mood nak keje hari ni..Fell like quitting my job.
 
I am suppose to be quite a strict person.Usually I don't find any problem with saying no but not untill I start working here.
 
She: Sharina, could you please help me by calling this client and follow up as well *pushing the shitty paper to 
          me*
Me:Owh..but i have other orders to settle also.
She:Well..I dun think you have enough.U see,if u want to hit your target you must atleast have 20
        orders..*looks at me in a way*
Me:Oh..Ok..*But i have more than 20 orders now*
 
This always happen also on last Wednesday when i was on MC for 2 days before.As i came in to the office she came to me and give me another order and u know what.I'm supposed to fill in 2 positions in a day..And they require someone who can speak and write japanese.I mean..hello..aku baru je balik from MC..obviously kau dah keje from hari isnin and job order tu from hari jumaat,apsal bagi kat aku?Oh ye..Lupe pulak..pasal position tu susah nak fill in and pasal manager tu org singapore.Habis tu keje2 aku yg lain bile aku nak buat? Bile aku tak buat keje aku..habisla aku kene maki dgn client2 seme..Dierang ni tau nak marah je..Lepas tu complaint kat boss
 
And today, i still can't find the time to do my work coz i had to fill in 5 placements for her position since she's on leave today.
 
Tell me..How shall I say NO? Coz the policy of the company is you have to help your collegue..But for somehow i feel used..