To be fair, Astral Projection isn't really something you find out about, try once, and succeed. The chances of that are *incredibly* low. It can take quite a lot of time and focus to really start doing conscious projections, unless you're one of the few who it just *happens* to, without there being any knowledge of the subject or any practice.
As Spiritual and Gaia have suggested, it's most likely a form of trapped nerves or bloody (a bit like pins and needles too), or it could be that you were entering the stage of sleep paralysis (it's often been said to feel like a led blanket is covering you from bottom to top). To be fair, our brains, minds and bodies can do some pretty weird stuff, that isn't related to any sort of spiritual happening whatsoever; don't give up trying though, but, also, keep an eye on your body. If it is trapped blood for instance, then that's not good, and can lead to troubles, which might mean you should start doing some stretches and the like, which could potentially help you relax more in the long run anyway.