I've had this as well. It's comon among folks with mental disorders and or sleep disorders. I's basically when your mind and body are out of phase and not coordinated in it's waking and sleeping. Some people experience buzzing or tingling sensations too when this happens. I did some research on this and basically everyone has sleep paralysis otherwise people will be always running around while asleep acting out their dreams, kicking hitting etc... The chemical signal goes out during sleep and paralyses our bodies so we don't act out our dreams and move alot. The condition we have that is called sleep paralysis should actually be called sleep phase disturbance or something like that because your concious mind wakes up before the signal for the body to move kicks in. Also people have trouble breathing sometimes because when in concious state we usually breath with our chests and not our belly. While sleeping it's opposite we breathe belly not chest. So when our concious mind wakens we try to take breaths with our chest like we were awake but we can't so we feel like we're choking but we're not. It's just our bodies are still breathing with the belly or diaphram. Sometimes this translates into hallucinations of someone sitting on you or choking you but it's not, you're basically just freaking yourself out due to not knowing how to transition from one breathing type to another. One tip I would have is to not force yourself awake and scare yourself. Recognize it and slowly try to move your pinky finger or toes first then you'll come out of it better instead of freaked out.

-Frank