You need to measure the sewn sections together before you then cut these strips into the blocks. Your 4- 2 1/2 inch strips sewn together measure 8 1/2. You then need to cut the blocks 8 1/2 inches instead of 8. You just forgot to add the seam allowance (1/4inch each side) to your cut.....
Easy to fix, just cut 1/4 inch off each side of your blocks to make then 8 inch square.
Good luck!