Did you take the thread out of both top and bobbin and re-thread? I doubt that the tension adjustment would have changed on its own while you were sewing. It's more likely that a tiny piece of lint or thread is caught in the bobbin area or between the tension discs. Threading the machine wrong can cause this problem too, but I gathered you had not re-threaded between the time it sewed fine and the problem?
Did you oil the machine? Those old machines have a lot of oil points, both above and below.
The only other thing I can think of is that possibly you were going so fast the treadle delivered a reverse 'blip'. I'm sure that's the scientific term for it. :? I know when I used a treadle years ago, I could make the machine sew backwards as well as forwards -- and not always with intent. It took me awhile to get the rhythm right so I didn't inadvertently reverse the sewing.