Sounds to me you are falling asleep way too fast, LOL. I suggest you to either get up after six hours sleep or stay awake for at least 5-10 minutes before you start SILD.
The sensation you felt is the typical phenomenon experienced when your body attempts to fall asleep when it detects long period of immobility. When you stay immobile for a while, your body, assuming it is sufficiently tird, will attempt to go to sleep. In order to make sure your mind is already asleep (as a safety measure perhaps), the body sends out a very strong signal literally shouting “I’m uncomfortable!”. If you respond to it by rolling or other movement then your body will seize to go to sleep; otherwise you body falls asleep and you experience some very strange sensations as it does that. This is in fact a very effective way to WILD but it requires an iron will and takes a long time to accomplish.
When the above happens, it’s always a good indication that you are doing SILD in a wrong way. SILD does not require you to stay still. You should be as comfortable as possible! If you feel an itch, just scratch it. If you want to roll, do it! Just do a couple of extra repetitions to compensate for the interruption will be enough.