We present optical and UV observations of the supernova PTF12glz (Soumagnac et al. 2018, to be submitted). This SN is one of the six SNe detected in the UV using GALEX, during a joint campaign by GALEX and PTF. It is among the most energetic Type IIn supernovae observed to date. If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF12glz was surrounded by about one solar mass of circumstellar material prior to its explosive death. PTF12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta is presumably masked by the optically thick circumstellar medium (CSM), the radius of the blackbody that best fits the observations grows at a velocity characteristic of fast moving ejecta rather than optically thick CSM. This phase of growing radius takes place before any spectroscopic signatures of expending ejecta appear in the spectrum. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF12glz. By modeling radiative diffusion through a slab of CSM with various density profiles, we show that an aspherical geometry of the CSM can result in a growing observed radius. In addition, this simple model allows to recover the decreasing blackbody temperature of PTF12glz. After presenting our results, we introduce SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature. SLAB-Diffusion can help analyzing future observations of interacting SNe showing the same peculiarity.