What Causes Development Of Subsequent Canker Sore?

I'm really scared right now. A couple weeks ago I got a sore behind my bottom lip and did research and discovered it was a canker sore. It went away in a few days, then a week or two later I got another canker sore. Then the next day a smaller one appeared next to it almost unseeable. Then I woke up this morning and next to the 2 canker sores is a black spot, like a pitchblack spot slightly smaller than the larger canker sore and the black spot is completely smooth, so it wasn't a piece of food or something else like I was hoping. I'm afraid it's something serious.