TV black shadow

Please someone help me. I have a Sony 61" TV KP-61V90. I just realized this problem today . My TV has a black shadow like discoloring on the top corner of the TV .I am not sure ,but I think it may be the color tube .Is anyone familiar with the description I gave ? If so do U have any good advice for me as far as repair .I can still ssee most of hte picture an clear it is just that shadow that looks like the beginning to a disaster I am worried I haven't had any problems in 4 years and I do not hear funny sounds or see lines of any sort just that corner. can someone help me?
Thanks Jody