My best guess is how you did your borders. Here's the trick I've learned to use to measure for borders. Measure the top, middle, and bottom. Add the three numbers, divide by 3. Cut the top and bottom border that length and stitch it on. You may have to ease or pull a bit to make them fit. Then do the same for the side borders. Measure each side and the middle. Add the 3 numbers, divide total by 3 and cut the side borders that length. Stitch them on.
If you leave the first border uneven, each border you add will make the problem worse. You will have more fullness to 'work in' than you'll want to deal with.
I'm a LA quilter. Trust me. Uneven borders are a nightmare!!
Becky
PS. I do like your quilt design and the colors!! Have a dachund friend who would kill for it!! lol!