Your body might be holding on to whatever you give it -- your 1200-1500 calories a day -- thinking it's in "starvation mode." Our bodies are resourceful little creatures, and if they're not being fed enough, they'll stop burning as many calories day-to-day (your metabolism goes down) so they can prolong your life in a famine. Maybe what you saw with the quesadilla (or at least the general idea -- don't know if bodies respond to just one quesadilla
) is that you upped your body's calorie intake, your body heaved a sigh of relief, and started revving its metabolism back to normal.
Point being, it's possible your calories are too low if you are that active... I'd suggest upping your calorie intake by a bit for a week or two and see what happens...