I don't enough about your situation, but I've been in numerous situations that involved me wondering why people were laughing at me, but I'll mention the first two that enter my thoughts:
I remember looking at an EastBay magazine for a basketball jersey during my Biology class in my 9th Grade year. While my classmates were talking about a certain pair of shoes, I happened to see those same exact shoes they were talking about, I said at random, "Those shoes are ugly!" And everyone started laughing. I wonder if it's because I usually said nothing in class, because I was always sleeping, but I'll never know.
Another moment was while I was in the 11th Grade, I believe. I was walking down onto the bus ramp from the school hallway, when some random female, never seen her before in my life, said, "Aye! You got stripes on yo shirt!" I replied, "I know," and walked along, minding my business as if nothing ever happened. I heard laughters and "OOOOOs", but I didn't know who it was geared towards or why. I figured later that the female was trying to trick me into ridicule, I suppose because they thought something was wrong with me, and I didn't "go for the bait", per se. Although, I didn't show it, I smiled inside
Anyways, the point I was trying to get at is, you never really know what it was. And it may bug the (expletive) out of you, and you may spend hours on end (at the least for some) wondering what you did wrong. I know it still does to me when I can't figure it out, just try your best to learn from it is the best I can say. Much easier said than done, but what can you really say about these things?