Which tension is off?
 

When I sew, my fabric curls up where the thread is sewn. Does that mean my top thread is too tight or my bottom thread is too tight? Thank you.

Do you mean at the start and stop of a seam or all along the seam line?

I'm not sure. Someone told me that if you have problems on the back of the quilt, it is the top tension that needs adjusting and if the problems appear on the front of the quilt, adjust the bobbin. I don't have a lot of problems with tension, but when I do I make slight adjustments and then test on a practice piece until I get a balanced stitch.

If you can see your bobbin thread, on top of the fabric, and if your top thread is laying completely on top of the fabric then your top tension is too tight.

You can also double check that you have threaded machine and bobbin correctly and that your needle is facing the correct way.

Also make sure that you have threaded the needle left to right OR right to left...whichever your machine requires.

.......... and only make one adjustment at a time ..... otherwise you will not know what worked.

I suspect the tension on your presser foot is too high. Try loosening that first. If you have a walking foot - try using it.

Perhaps it is not tension but the stitches that are too tightly together. Try lengthening the stitches up a notch and using a walking foot if the fabric is heavier than usual. Or it could be the top thread is caught on something while threading the machine. That happened to me once. The thread wound around the spool on the last bit of winding a bobbin and I didn't realize it. That caused a drag on the thread and a top curl of the fabric.

I'm using a walking foot already. My bobbin stitches aren't showing. My fabric is curling into a 'C'. I can pull the stitches and straighten out the fabric but I lose my stitches on each end of the fabric. It just started happening five days ago. I'm not doing anything differently than I did before. I'm just piecing two pieces of fabric together. I have 50 wt. thread in top and bobbin but one is Presencia thread and one is Aurifil. Does that make a difference? Should it both be the same?

I am not sure, but when my machine seems out of wack, even though I am still sewing the "same" material, same thread, etc.. I clean my machine thoroughly, you know with the vacuum and all. I then rethread and sew again. In all cases this has fixed it. Think it has something to do with a build up of lint in top and bottom thread channels.