Do you know what the stipules are? They are the little spikes that form at the node during veg. They almost look like a pistil, and get mistaken for them all the time. When its about to start flowering, those stipules will go from sticking up, to shooting out directly to the side. When they do that the preflowers are about to show.
Can you get closer shots at the nodes? She might be flowering, when you say day 28 what so you mean exactly? Day 28 from sticking the clone into dirt?
That plant is nice and healthy, looks good! I heard blue mystic is the shit.