Barb in Louisiana

I get something similar if the bobbin doesn't keep an even tension on it when it is winding. Does the thread look smooth on the bobbin itself? If there are any loops it could be the problem. You can either rewind it on to another bobbin or use it to sew with as the top thread. If your machine sewed properly before this bobbin, then I would suspect the bobbin winding or lint. Or the bobbin could be in backwards, or there could be lint somewhere in the machine. It is amazing that lint under the bobbin case can cause those kind of stitches.

But....the top thread is supposed to pick up the bobbin thread and pull it up some to tighten it and make a seam that lays flat. Did you recently change the thread on the top? Try re-threading the machine on the top. Typically, if you see the problem on the bobbin side, the top thread is the problem. Maybe it didn't go through the tensioner. Maybe you missed something in the threading path? .. mess with any tensions without taking pictures or writing down where you were when you started.

Sometimes, but not often, the thread does not like the fabric you are using. Try a different fabric with a simple seam to see if this could be it.