$\mathrm{ABCD}$ is a rectangle formed by the points $\mathrm{A}(-1,-1), \mathrm{B}(-1,4), \mathrm{C}(5,4)$ and $\mathrm{D}(5,-1) . \mathrm{P}, \mathrm{Q}$, $\mathrm{R}$ and $\mathrm{S}$ are the mid-points of $\mathrm{AB}, \mathrm{BC}, \mathrm{CD}$ and $\mathrm{DA}$ respectively. Is the quadrilateral PQRS a square? a rectangle? or a rhombus? Justify your answer.