Saying "No" !!

Today, I realized that I can't say "no" to anyone that easily. I need to do my work called (deployment of applications) once others complete their work (coding). This is supposed to happen before 5 PM ( rather 4 PM). X always comes with unofficial requests to re-do the work. He needs to follow a process and get approval from his manager etc .. but that would be a black-mark to him and would take a lot of time. So I wasn't strict and did all I could to help him. Once, he foolishly let out an unnecessary/false comment on our team's work. I was pissed off for a moment but then just left it. I told this to my roomie. I told her that the next time I'll say "no" to him. My other team mates started to be strict with him after this incident and he had no other option but me. Surprisingly, the next time he called me, I totally forgot everything and said "OK". This is still continuing. I got a phone call at 10 PM from him today. Me : OK... let me know once its done and don't take too long. I need to wake up early tommorrow to be in office early. X : Sure. My collegue (and room mate):"Why are you so forgiving? He doesn't deserve it. You are already tired" She was right. I need not have stayed back to complete his work. After all, noone is going to know that I helped him. Even he doesn't understand that. I started thinking on why I didn't refuse and say "no". We are 3 people here in our team. I live near to workplace but one person (say Y) has to commute for an hour to reach office. They both live with their families and I'm alone :(. So, I make it a point that I look into all requests at night. Whenever he requests me to come early instead of him, I'll happily say "ok". But oneday I wasn't feeling well and left early from office to get some rest. They both didn't take care of things even that day. I got up did some work for an hour went back to sleep..30 minutes later.. another task.. I was really frustated and told to my roomie..I'm not going to do anymore work today. Immediately, Y called me " Pavithra, we have some work starting at 11 PM tonight till 2-3 AM and since I'm doing some work, can you take care of the rest." I said "Ok". My roomie got too angry on me. But I thought, Y had to play for a cricket match early next day and the other person had planned to go to a theme park with his wife, early next day. That was the only thing running in my mind..I knew there were 5 tasks and I accepted to do 3 which would take me 5 hours from 11 PM. Why didn't I tell him I'm not feeling well so I can't do this? Was it my "ego" and I didn't want him to sympathize me? In my prev project, I had to work for atleast 18 hours a day of which my work would have been not more than 6 hours. This was the only credit that I got in my appraisal letter - "She never refuses to help others in the project. Personal Efficiency -10/10". Why do I have this disease of always saying "yes" for requests that too happily with a "smile" as my clients point out? I never crib abt doing work except when even after doing so much..noone recognizes it and points a finger at me. Whenever I go to my relatives house with my parents, and if someone offers me a food or drink, I'll be the only person to accept it (inspite have having a full stomach from visiting a series of relatives the same day :-)). I don't know to refuse. Even if there are certain things that I never want to do, I do them. My mom tells me "Un kita kaariyam saathikarathu rombha easy..konjam paavamaa, soft ta sonna pothum". May be true, I get pissed off if someone orders me to do things and I'll fight till the end to protest, but the same thing told in a loving manner I'll do it at any cost. I'm not sure if its an entirely bad thing. It's good to be liked by everyone. But then I need to realize that I should not be fooled emotionally. X taught me a lesson today not to be a simpleton ..but will I be able to follow it ?? God only knows !!