Oleanders branches can become droopy if the blossoms are large and to heavy.
If you see not other signs of plant health, this is most likely the issue.
Pruning can help you correct and tidy up the plant.
If you seen any continued decline in health of the plant it could be Oleander Leaf Scorch.
Here are some links for you.
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/pruning-oleander-shrubs.htm
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/bacterial-leaf-scorch-disease.htm