That's either slight nute burn from too much n, or possibly p- since it is starting to flower. If you think it might need more phosphorus because it's flowering, add. If it's too much it will work its way out in time, just back off a bit.
That would be, or would have been a great plant to super crop its top which is simply bending the top downward without snapping the top. That pushes the lower branches to shoot up, but also your top will recover and flower as normal. Once flowering is under way the options become limited.