your thread nests look as if that is where you started and stopped? caused by not bringing the bottom thread to the top before starting to stitch.
to avoid this at the start (every time) put the needle down, hold onto the thread (that is through the eye of the needle) and bring the needle back up. the bobbin thread will be looped around the top thread., hold both threads as you begin to stitch. after a few stitches you can clip those threads, some people stop and tie them into a knot, some people thread them onto a needle and poke them into the center of their quilt sandwich. what ever you choose to do with them is your choice, but bringing them both to the top will prevent those nests on the bottom :thumbup: