I've been changing colors on the top thread, to match different areas of the quilt top, and using the same color in the bobbin because it blends in well with the backing.

I took a break from that and was FMQing with a different thread and bobbin, foot, etc.

I put the feed dogs back up, changed back to walking foot and changed needle, thread and bobbin.

Now my bobbin thread is suddenly pulling up to the top where it is visible, it looks terrible.

Do you have any idea why?

Rule of thumb is that if the problem is on the underneath of what you're sewing, the messed up part is on the top (tension, threading, needle, etc). If the problem is on the top side of what you're sewing, the messed up part is underneath (bobbin, bobbin tension, lint, bobbin case). Hope this helps.

Loosen your tension? Make sure (esp. if you plan on using different threads on top and bobbin!) to test on a sample sandwich!

I suggest you totally rethread the machine. That helped me before and was way easier than dealing with tension adjustments.

Try changing bobbin thread tension. Usually a small screw in bobbin case.

I agree that the tension sounds like it is too tight. I would work on that first.

The tension is set exactly as before, same quilt as before, same thread as before. I already quilted a bunch on this quilt. That's why I'm wondering why the change now. Maybe I need to take apart the bobbin case and clean inside, but I never have had to adjust any bobbin case screws.

I was using inferior thread in the bobbin when I was practicing FMQ so maybe it left a bunch of lint?

I'll try rethreading everything, already tried new needle.

If the bobbin is wound with different thread it could have gone on the bobbin at a different tension. I wouldn't adjust the bobbin tension, but I would try moving the top tension and see it that makes a difference.

When upper and lower thread tensions are balanced, knot between top and bottom threads is hidden between fabric layers When lower tension is too loose (or upper tension is too tight), knot is visible on right side of the fabric When upper tension is too loose (or lower tension is too tight), knot is visible on wrong side of the fabric. Perhaps you are now using two different color threads and it is more noticeable. Try using the same color thread and see if you stil notice the thread showing on the top. Yes, and clean out your bobbin area it sure could be that dust bunny in there.