I have it too sometimes, it's quite normal, don't worry about it.
When meditating you are bringing up very large amounts of energy, in the beginning your body isn't really used to this yet, so you can experience a wide variety of strange and unusual feelings, one of which is twitching.
To stop having this sensation or the twitching in general, carefully guide the energy you rouse up during your meditation and make it flow calmly through your body.
It can also indicate a potential blockage in your meridians at the place of the twitch, which makes it even more important you try to calmly guide the energy and make it flow like a stream. Note, you aren't doing something wrong when you feel a twitch, but it usually indicates either a blockage or is a result of having more energy at the time than your body is used too.
Doing Hatha yoga really helps strenghten your body and increase your tolerance for large amounts of energy.
