Hi Phyony! Welcome to the forum!
I think you were not so far from a WILD. Your technique is perfect but, in my opinion, you waited too long. You went through the hypnagogic state (the “silly pictures” state) and when you felt your legs were melted, you were asleep (even if you felt you weren’t). I suppose you had to do something like rolling (mentally) out of your bed in order to go into a dream. But I’m not a WILD’er and I can’t give you very good advices.
The symptoms you describe are typical of WILD (heart pounding very fast for instance). About pains, I don’t think it was because of your position. It seems they are quite common in the first tries, but they disappear with experience. Just don’t focus on them. This subject is discussed in the BIG WILD topics, but I know it’s rather hard to find the good posts in all these pages…
Most of the WILD’ers lie down on their back, but it’s also possible to lie on your side. And as I said to you above, it’s rather difficult to know whether you’re asleep or not, because in many cases, you feel like you’re awaken though you’re asleep. I think that when you feel that you’re dreaming your hands, arms or legs position, you’re asleep and you can try to enter the dream world.
If you just tried a couple of times, it seems you’re really gifted with WILD (I agree on the fact it’s similar to OOBE). You’ll probably will have your first WILD very soon. I hope that someone more experienced than I am will answer accurately to your questions.