To Say Something or Not

In the past I have worked in the positions are really the catch all to the terrible jobs that no one wants to do. This job that I currently have is no different. I am assigned jobs that no one wants to do but everyone has an opinion about. It is so frustrating to go through all this work and be told to do it and then people get involved and change things. What is more frustrating is to not be told that things are being changed. In the three months that I have worked here it has happened in almost ever project that I have done. Do they really think that it is fine to go behind my back and do this? So...I am struggling with standing up for myself and saying that it is disrespectful and rude to do this or to sit by and allow it to happen time and time again. This was one of the big issues that I had at my previous job and I was so miserable there. I do not care at all about this company but still would like to stand up for myself. What to do?