Can you get an astigmatism from working in front of the computer for too long?

NO. Astimatism is a type of refractive error like myopia (nearsightedness) or hyperopia (farsightedness). It is basically determined by the length of the eye and the shape of the front surface (cornea) of the eye. Sustained focusing for near may induce a progression (higher prescription) of a myopic refractive error but not asigmatism.