Q1: What differentiates humans from machines?
Answer: Many things differentiate humans from machines, but perception-wise, what differentiates humans from machines is conscious perceptions: humans have conscious perceptions but machines do not. Conscious perceptions are perceptions that we can be conscious of.
This means that we know what it is like to have those perceptions, such as we know what it is like to see the color red, to hear a musical sound “C”, to smell the rose odor, to taste the sweet honey, and to feel the pain when we have the perceptions of the color red, the musical sound “C”, the rose odor, the sweet honey, and the pain, respectively. (ref 1, ref 2, ref 3, ref 4, ref 5)
This is in contrast to unconscious perceptions, which are perceptions that occur unconsciously and that we cannot be conscious of. That is, we do not know what it is like to have those perceptions, such as we do not know what it is like to have blood sodium level at various levels, to have blood sugar at various levels, and to have various blood hormones at various levels, although we are unconsciously aware of them and react to them all the time.
Machines’ perceptions are like our “unconscious perceptions”. Machines just acknowledge various signals and react to them (such as acknowledge the high pressure on them and react to them, in your example) without knowing what it is like to have those signals – the knowing of what it is like to have those signals does not occur in machines. This is similar to what happens when we have unconscious perceptions, in the preceding paragraph.
Now how do we know that a machine does not know what it is like to feel signals? The answer is to know what it is like to feel signals is an additional phenomenon to the phenomenon of feeling the signals. The additional phenomenon of knowing what it is like to feel signals needs an additional circuit. Present-day machines are built with circuits to just acknowledge various signals, so they just acknowledge signals as they are built to do. They do not and cannot do something more or less than what they are built to do. (This can be checked by checking all bits of data in their systems whether they represent something else that they are not pre-designed to represent. And so far, it has never been found that there are unexpected bits of data in normally-functioning computer systems.)
To recap, to know what it is like to acknowledge signals needs an additional circuit to create the additional phenomenon of knowing what it is like to acknowledge signals. But, nowadays, there are no such circuits because we don’t know how to build such circuits yet.
Q2: But what are these feelings (like the pain you feel and can't stand)?
These feelings are our conscious perceptions, as described above. Some philosophical literature calls them qualia. (See ref 1 - 5 above.)
Q3: Could machines be able to feel (specifically pain)? How?
Machines can feel (e.g. pain) as we do if they can have conscious perceptions as we have. But to have conscious perceptions like us, they must have circuits that can create conscious perceptions (in addition to circuits that just create unconscious perceptions, or acknowledgment, of the signals). The problem is, at present, we do not know how to build such circuits yet. (see section 6.6.2 this chapter) And it is still controversial whether, theoretically, such circuits are possible in machines.