If it's on the back, it's the top thread that is causing the problem. As someone suggested, rethreading the machine would be the first thing to do. Then if this doesn't solve the problem, work with your top tension.

Thank you everyone for your comments to help me solve my problem. I re-threaded and changed out the needle but had no luck, still loops. So I will check my tension section next.

if loopies on the back it is the top tension that needs attention. If they are on the top it is the bobbin that needs attention.

That happened to me recently when I put in a new bobbin. I use only prewounds but something must have been different. I had to up the top tension quite a bit. I was afraid the thread would break. It did not and the stitching was fine after doing that. I prolly tightened it 2 full turns on the top. I was getting VERY annoyed.

Is the nesting from the top thread, or the bottom. If it's bottom thread, this may be caused by the bobbin spinning too fast with the loose thread getting caught up. To fix it, I just cut a 'washer' from cotton fabric to insert into the casing before putting the bobbin back in. Fixed all my problems. Hope this helps

I had to laugh. I remember reading this thread and yesterday and twice I had the same thing happen on my Bernina. Both times, I re-threaded the top thread which had been replaced and everything continued smoothly.