I always advise a quick tension check when you switch to a different spool of thread. The color and type of thread, even the brand of a certain type of thread such as silk, the humidity, a dull needle, all will affect the top tension and how the stitch looks. It only takes a few minutes to do a bit of off-quilt stitching on a sample to see if all is well.
One of the first things I advise students in every class I have taught is to check the thread pathway before beginning to quilt. From the spool all the way through to the needle's eye, make sure nothing is catching or interfering with smooth, even flow of thread through the machine.
With the presser foot UP, pull the top thread through the needle. It should move smoothly and easily; the needle shouldn't bend. If there is a catch, or the thread is not coming off the spool evenly, you will feel it right away.
Bless your heart, try a cone thread holder....!
Make sure all thread guides are used. If you have issues with the top spool pulling too tightly and creating tight tension, try a cone thread holder instead of whatever your machine provides. In the photo, above, you can see it sitting behind my machine to my right, out of the way.
A large cone of thread sits on the base spindle, unwinds gently with no pulling or tugging, goes up through the metal loop, then goes down to the thread guides on the machine. It's the best method to deal with large cones or unruly threads such as invisible monofilament, but I have used it for normal spools because it works so well. It's a nice option to have if you sometimes have trouble with upper thread tension.
However, trouble might be in a place you never check, the spool itself. In her blog today Jenny Lyon discussed her adventure with figuring out why the top tension suddenly was very bad: quiltskipper.com
Jenny has many good ideas and posts about home machine quilting, design, problem solving, etc. on this blog, and you might want to check back often, and also enjoy her creative and beautifully done work.
Meanwhile, may all your spools be well behaved! Take a break as I am from dusting the house, or shopping, or baking, and do a bit of quilting. You will feel better for it.....