Generally, thread issues that appear under the fabric are the result of an upper thread problem. What you describe sounds like bird nesting or thread nesting and are usually a result of: mis-threading, thread not being fully seated in the tension disk, and/or upper tension too loose.
Remove the upper thread. ALWAYS raise the presser foot. Rethread from the beginning. Verify the thread path is correct. Set the upper tension to the midway point between high & low numbers (although this will probably need to be tweaked for your particular machine). Retest your machine. Adjust the upper tension until both threads meet in the middle of your fabric.
FWIW, mis-threading may not be your mistake. I have an older machine that the take-up lever is open at the top. Sometimes, usually when sewing fast, the thread will literally "jump" out of that take-up lever and...instant thread mess... So, if this sort of thing occurs frequently with your machine, try to narrow where the problem originates. With my machine, I "manufactured" a method of closing the opening in the take-up lever. Voila! No more jumping thread!
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Birds nest under the fabric big loops of thread top side looks good...
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