I"ve had this happen, but not in 2-3 months. I put a watch into the vault at the bank for a year or so, and when I retrieved it, it had the same stress cracks. Watchmaker explained that plastic crystals dry over time and lose their elasticity. They even shrink a little. So depending on which mounting system design the crystal is, stress causes the cracks to appear as the crystal becomes more brittle over time. No idea why yours happened in such a short period, though.
Had the opposite problem recently. The crystal on my vintage Omega just fell out one day. It had dried and shrunk over the years until it was just too small to stay in place.