Like previous poster said you are still fine to lay on your back because the weight of the baby is not enough to put pressure on your spine yet. My dr actually let me lay on my back til I was 24 weeks... Check with your dr though because that is always the safest way to go so they are aware of what you are doing. But yes I have had dizzy spells and especially after laying down if you sit up to fast the blood has sort of escaped your brain and so it takes a little time to get back up there to feel normal again. Just sit up slowly when getting up from a laying down position. Of course by the time you are further along it takes a lot longer to sit up anyways... :)
its normally ok to lay on ur back if you're less than 4 months. did u sit up fast? move slowly and increase ur protein and decrease ur sugar intake and u should feel better :o) if it keeps up all day I would call doc though. feel better! :o)