Light-matter interactions are powerful tools that seamlessly allow both functionalities of sizeable bandgap modulation and non-invasive spectroscopy. While the border between modulation and detection is often assumed to be sharp and well-defined, there are experiments where the boundaries fade. Here, the interplay between bandgap modulation and non-invasive spectroscopy is measured and explained in the case of resonant perturbative nonlinear optics in an atomically thin direct gap semiconductor.A clear deviation from the typical quadratic power scaling of second-harmonic generation near an exciton resonance is reported, and this unusual result is explained based on all-optical modulation driven by the intensity-dependent optical Stark and Bloch–Siegert shifts in the ±K valleys of the Brillouin zone. The experimental results are corroborated by analytical and numerical analysis based on the semiconductor Bloch equations, from which the resonant transition dipole moments and dephasing times of the sample are extracted. These findings redefine the meaning of perturbative nonlinear optics by revealing how coherent light-matter interactions can modify the band structure of a crystal, even in the weak-field regime. Furthermore, the results strengthen the understanding of ultrafast all-optical control of electronic states in 2D materials, with potential applications in valleytronics, Floquet engineering, and light-wave electronics.