Usually if the upper thread is looping on the bottom, it has to do with lack of tension on the upper thread. The next time this happens check to see if the upper thread has changed:the arm that goes up and down directly above the needle, that you have to thread the thread through, did it fall out of there?
when you rethreaded the upper thread last time, was the presser foot up? It has to be up when threading, presser foot up = tension discs open /presser foot down = tension discs closed, when you rethread it next time kind if give it a tiny tug back and forth like flossing it into the discs
Because it doesn't happen right away, it sounds more likely that it fell out op the thread "uptake arm" (just rememmbered what it was called).