Little.girl, he may have talked to you for twenty minutes too if you'd been friendly enough to smile and say "hi" when he looked at you.
How would we know if something is going on between them? A 20 minute chat between people of the opposite sex is insignificant. He didn't deserve to be ignored.....you're just being jealous and over-thinking. However after you ignoring him, it's likely he'll think you're stuck up and not bother with you in the future.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.