  The low-lying energy excitations of 2-flavour QCD in the chiral limit
confined to a small spatial box ($\delta$-regime) are that of an $O(4)$
rotator. In this work, we calculate the corrections due to the presence of a
nonzero quark mass up to NNL order by means of dimensional regularised chiral
perturbation theory. The final result for the energy gap of the system only
involves the low-energy constants $F$, $\Lambda_1$, $\Lambda_2$ and $B$.
