Sounds like maybe it is the accumulation of small errors? That consistent seam allowance is a biggy and can get skewed by thick intersections.
Otherwise if the project has been actually hanging up for awhile, the physical stretch of the fabric can add up!
Depending on the style of construction, the simplest resolution is to simply square off what you have by declaring a straight line and going from there, trimming off the stretch. And sometimes we just have projects that finish in less standard shapes...
So hard to determine without pictures. Can be hard enough to figure out how to post pictures, but while some of us have insecurity issues, trust me -- the time to share a picture is with a problem and not a perfect project! We love the perfect project pictures too, we just have a whole lot less of them.