I've experienced this too, and it's not the same as the practice of tummo, tummo has a LOT of elements that are more than just feeling heat. Though that's not to say there's not a connection between the two, meditation and anything else we do have some effect on our channels and winds, as do inner yogas. But I think this is actually connected more to digestion and circulation than anything else. When your breathing relaxes and slows down and deepens, your digestive organs receive more oxygen and circulation. Even without going into meditation, take a moment and breathe deeply into your stomach a few times, and it should feel a bit warm in there. That's not to minimize your practice at all though, and even if that is all it was that's not necessarily a bad thing.