CE amplifier voltage gain
In small signal modeling of a bjt transistor, there is an intrinsic emitter &quot;resistance&quot;, \$r_e = \frac{25mV}{I_E}\$. You need to add this to your external resistance \$R_E\$. \$r_e + R_E\$ is the total emitter resistance to use in your gain equation. When \$R_E\$ is small, or 0, the effect of \$r_e\$ on gain becomes significant.
(A completely unrelated side note: Although the phase of your output is inverted in relationship to your input, I think it is bad practice to carry the negative sign into your gain. When gain is described in decibels, negative gain is loss. Just a suggestion. Your teachers may strongly disagree!)