I am sorry your are in this situation. It sounds like an anxiety attack. It is a normal response to trauma and stress.
If you are faced with the possible death of your mother from cancer, I'd day that is pretty traumatic.
Since she said that impatient thing to you...I imagine she is in pain, on medication or having some discomfort of course...but, you may have sort of come up with something to focus attention on yourself and away from her...there are many, many reasons people do this, especially if you are the baby of the family. That is what is normal for you and you are trying to get back to "normal" right now.
Time to do a little maturing...which is also normal.
It is time for the focus to be on her and for you to be strong so she can fight her own battle and not yours this time...I am sure she has been there to comfort you in time of trauma, stress and illness...right now she needs you to fill that role. It is very normal.
But, if you keep obsessing about it and it keeps you from functioning normal day to day stuff...then get some professional help to get strong again.
Take care, and you are not crazy.