Sometimes people with unipolar depression will have short duration happiness due to stimuli in their surroundings during a depressive episode. This is considered atypical depression. If it’s short like you are describing, it is most likely that. It isn’t hypomania.

Actually, to further this, depressed people will often have a short duration up right after a depressive episode. However, this isn’t hypomania either. This is just the result of a psychological leaden brick being taken off your chest.

Even people without a mental illness will have short duration mood fluctuations. It isn’t at all pathological.

Personally, while I’m not a doctor, I would not worry about this because it’s not long enough to have a negative impact on your life whatsoever. If your depression treatments are working, then you’re probably in good shape with your current care. Breathe a sigh of relief!