From the datasheet:
8.1.5 Catch Diode
Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode must be placed close to the LM2576 using short leads and short printed-circuit traces.
Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best efficiency, especially in low output voltage switching regulators (less than 5 V). Fast-recovery, high-efficiency, or ultra-fast recovery diodes are also suitable, but some types with an abrupt turnoff characteristic can cause instability and EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60-Hz diodes (for example, 1N4001 or 1N5400, and so forth) are also not suitable. See Table 3 for a Schottky and soft fastrecovery diode selection guide.
It would surprise me if your "regulator" is even regulating without the catch diode.
Whatever your circuit is doing, it isn't operating as a proper buck converter. You should stop messing with it. Install a proper diode before you use it again, or else you may need to replace the chip as well as install a diode.