Codeine causing vastly different effects than Semisynthetics?

[BlackWarGreymon]

I recently was prescribed Codeine and one effect I am questioning is the overwhelming feeling of heaviness in all my body 20 min post oral administration. I do get some warmth just before this like an adrenaline rush then the heaviness with no nausea but I do get some doubling of vision in higher doses like 360 - 410. My question is why do I not experience this with any of the Semisynthetics like Oxycodone, Hydrocodone, Propoxyphene, Hydromorphone. Those give a feeling of slight muscle relaxation but more of a euphoric mood and some warmth. Codeine seems to cause some euphoric mood and warmth but this muscle relaxation or heaviness seems to be overpowering the other effects. Anyone have an explanation or similar experience or can give me some advice on this effect?