This looks like a problem with your cutting when you are cutting your strip sets into the rows of diamonds. It looks like your cuts gets progressively more off from the desired angle with each cut. If the angle is getting larger with each cut, each strip will be a bit longer than the one cut before it. (And conversely, if the angle is getting smaller with each cut, each strip will be a bit shorter than the one cut before it.)
The video link that Rhonda K posted, above, shows how to use the lines on your cutting mat to get your strip set positioned on the 45 degree angle, and then use the vertical lines on the mat to position your ruler, and thus get an accurate 45 degree cut with every cut. (I wish I'd seen this video before making my first Lone Star quilt!)