Some students giggle at me while I teach! I am an associate lecturer and its my second term with the undergraduate students. 

I noticed some of my students laugh when I teach. Its the most annoying thing I ever felt in my academic life so far. I always think, why some laugh while others appreciate my way of teaching? Its very puzzling. 
At the end of my first trimester I received a very positive feedback from the majority students. But just those couple of giggling students ruin my whole confidence. 

I don't speak funny and I look average too but why why that happens with me? 

Is it normal in academia? Did this happen to anyone else? And how I can deal with them?
In my experience, there are some students who just cannot take class seriously. They may not even be laughing at you. Rather, they may be talking with a classmate and find what they are discussing amusing. They may not be paying attention to you at all. When they take a test and do not do well, they may react by thinking that is funny. If it bothers you enough, you should talk with the students after class and explain that their behavior is distracting to you, and probably to their classmates. This might appeal to whatever sense of fairness they may have. If they continue to be a distraction for you, a pointed remark about immature behavior may be appropriate, again after class. You may, if you wish, indicate that their behavior shows a lack of respect for you, the course, and their classmates.

I would not call the students out in class. I would also consult the department chair for suggestions on how to handle the situation.