I do the same thing, and have found that it is the muscles reactions from being tense and the body's reaction from being over tired. Try some relaxation techniques before going to bed, and listening to soothing sounds as you fall asleep, it may help with the problem.
Hi, welcome to the forum, most people have experienced a sudden jerking movement upon falling asleep, often accompanied by a subjective sensation of falling or twitching. Such occurrences are known as benign hypnic myoclonus or sleep starts.
Rarely, night starts can become unusually violent, very frequent, or repetitive, and confused for seizures. The phenomenon is restricted to sleep, and always or virtually always in the transition between sleep and wakefulness, and is not associated with other phenomena.
To conclude, you need not worry; it will not harm your body and will wear off over a time. Maintain good sleep hygiene, be positive. I suggest you to consult sleep specialist for more information. Take care and regards.