When I lie down on my back, it feels like something pressing on my throat, choking me. Feel front of throat, large hard knot. Ideas, cause?

See a doctor. Unclear by history, but depending on where the lump is, it may be an enlarged thyroid (if at bottom of neck). Could be any number of things, possibly concerning in light of size of mass and firmness, especially if you smoke. If your neck was fused, a pressure sensation in the throat is very common, and you could have some fibrosis / scarring (small price to pay for healthier spine).