Yeah it was cum. Your body probably ran out of cum for a little while, because
A) You could not be drinking enough fluids to make the semen
B) Your body has just started puberty and it doesn't make that much at a time
It's probably a combo of both, but as your body matures it will produce more cum and you'll be able to ejaculate some even if you masturbate 4 times each day for 3 days!