Sharing Our Innermost Thoughts

Earlier I was an assistant so I did all the work asked by my senior but now I’ve been promoted and have my own responsibilities but still my senior and some other people treat me as an assistant and occasionally ask to do some of the work. If it’s a small work then I just help them for a little while and continue with my work but recently my senior been asking me to do almost all high functioning work which sometimes irritates me and hamper my work also but I’m unable to say no to them. I’m afraid they will say just because I got some new responsibilities I’ve become egoistic and think of myself very high. Even if I don’t have any work I don’t want to do their work because for I’ll have to do extra hardwork in searching for all datas to complete that work. Today my senior was really busy so whoever has some work with him ask me to do that but why should I do that I don’t have any knowledge of that thing so they want me to gather all the datas and complete the work. Also another assistant has been appointed under that senior but he still ask me to do his work because his assistant is still learning but what about my work. Can someone tell me how to say no to them without offending??? Please someone help me.