"notice that the content of your conciouness is the only thing that's certain".

Tell that to someone with multiple personalities. Tell that to someone who has brain injuries reducing them to a vegetable. Trauma applied to the brain directly and (to an extent) predictably causes changes to the consciousness, making its contents anything but certain.

I think you've misunderstood what I meant. For example, when I see a lion, I can be certain about it. It may be a hallucination and suddenly disappear but that doesn't change anything, I really saw the lion.

No you saw something that you interpret as a lion. Thats a very different thing.

I was refering to the experience of seeing, to the state of my mind. In other words, when I feel pain, I know with absolute certainity that I currently have this unpleasant feeling that I call pain.

Which does not mean that what you experience is pain.

Why?

Your last statement is only true until it isn't.

(Imagine a device that projects some unknown field at you and can predictably manipulate your subjective experience; it must have a sufficiently complete model of your consciousness and it is difficult to say much more than it is unlikely to be possible)

It sounds like you're a philosophical idealist. Have you read Berkeley? He argues that it's insane to believe in a 'material' world, since we can never experience that nor have evidence of it. What we experience are ideas.