Try changing to a different bobbin. Sometimes bobbins get warped. With a metal bobbin, this can happen easily if the bobbin is dropped. Plastic bobbins can get brittle or distort from being over-filled with thread.
What machine do you have? Does it have a bobbin case? A dropped bobbin case can become distorted also.
Because the bobbin thread is looping at the top, it sounds to me as if the bobbin thread is not getting sufficient tension.
Yet another possibility, if you have a bobbin case, is that a tiny piece of lint has become trapped in the bobbin case tension mechanism. This is easily removed with a pin. This video by Jamie Wallen shows how to clean the bobbin case tension (for machines that have a bobbin case! does not apply to top-loading bobbins!):
https://www.youtube.com/watch?v=Q1mRhcquZTM
Edit: One more possibility is a poorly-wound bobbin.
Edit 2: If your machine uses a bobbin case and the case is distorted, you will need a new bobbin case. If your machine uses a top-loading bobbin, double-check your threading of the bobbin thread path and also check for any lint that might be caught in the bobbin tensioning mechanism. The latter can require a magnifying glass and tweezers.