I felt strong vibrations

Today, I woke up and felt strong vibrations throughout my body for a few seconds. It felt like I would fall off my bed any moment, yet I just lie there not moving a bit. At the same time, I felt the presence of my friend right aside me (which I just saw in the dream I had right before I woke up), but I was alone. I wasn't scared at all, as I immediately remembered reading about such experiences in WILD related texts and thus I knew it be safe. I never experienced such a thing before, eventhough I'm used to do WILD.

I wonder, why does this happen? And why didn't I ever experienced this before?

It's just your standard sleep paralysis (SP). You're usually unconscious/asleep when it happens so you don't normally experience it with awareness.

If it's what I think it is (SP and HI), then it's a perfectly normal albeit rare occurence. It's just your mind waking up before your body has fully awakened. Until I got serious about lucid dreaming, I had only experienced it a handful of times. After I started using techniques like MILD, however, I started to get a lot more of that stuff. I still frequently "wake up" in the middle of vivid HI.