Hey Joh An, welcome to the DhO.
By "self inquiry", I'm assuming you're talking about asking "Who am I?", in which case it's perfectly normal to notice anxiety or this sort of gut-level fear without any recognizable cause. It's typical for most people, at least at some point, regardless of which method you're using to experience this so don't worry about it. I'd also advise against going to your doctor or seeking medical advice
unless it really begins to interfere with your ability to go about your business as normal; doubt, uncertainty, fear, anxiety and discomfort will come up again and again because you're directly investigating that which
seems to be "you", it can be less than pleasant sometimes but is ultimately worth it.
A good way to deal with these feelings and sensations is to just be there with them as they happen. Don't try to fight them, change them or stop them, just let them be and recognize that they never last more than a matter of seconds (even though sometimes that doesn't seem to be the case) and how, no matter where you look or how you look at them, they aren't "you", they don't belong to a "you" and can't be said to contain or imply any sense of "you".
in some way i can realise why there is fear and whatever, i kind of realised that i have to die to be what i am
Good insight, but it's worth knowing that "I" never even existed in the first place and so "I" don't need to do anything. If you continue to investigate in the same way you have already, it'll become clearer through direct experience that "I" was only ever a thought, no different or any more special than any other thought that happens, and you'll find it easier to let go.
Hope that's of some help to you.