Early flowering

It's an autoflower, so possibly completely normal. However, you are transplanting and topping an autoflower... some may not take kindly to such actions and stress it into flowering early. No idea on ratio there as some ppl top/transplant their autos with no probs. but stress can cause early flower, nonetheless, and autos are a bit more sensitive due to whatever reason if not related to being on a veritable genetic timer. Preflowers should show up fairly early. figure an autoflower is similar to a photoperiod that gets 1-2 weeks of vege growth before flip.... some "XL Auto" varieties may be longer before they hit flower full throtle.