How long did you practice for? If you've been practicing for a while you probably just taught yourself to write with your left hand, and if you keep going you can probably write better using it.

If this was your first time practicing, is this the first time you've asked yourself this? If you've been wondering the same thing even as a kid you might have just lost control of your left hand bc you haven't used it very much, and again if you practice a bit you're gonna regain your ability.

If this is the first time wondering this you're probably just better at using your left then most people but still not ambidextrous. (this might also apply if you asked yourself this in the past but I don't know if there is any way of knowing for sure)