I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
I'm not sure why you're plant dropped its flowers but it is not unusual for oleanders to go in or out of flower periodically. If the foliage looks fine you are probably doing all you can and should for the plant. It may have been that the plant in the past went through a brief spell with either dry soil soggy wet soil for too long causing the stress followed by a flower and leaf drop. At this point in time I don't recommend you do anything other than provide moderate watering during extended dry spells.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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