Can Botox Cause Water Retention Under Eyes?

Botox does not cause swelling. And, unless it is very dilute, there should not be much saline either. Additionally, the small amount of saline used with the Botox should be absorbed with in a few hours at most. If you provide pictures, we may be able to give you more advice.

This is an uncommon complication but I have seen it before. I would massage and it should resolve in a couple of weeks. If not , you may have to deal with it until the botox wears off.

The fact that the lower eyelid became puffy one week after your Botox injection makes me doubt that this is due to edema. I suspect that this "bag" could very well be from pseudo-herniation of the obicularis oculi muscle due to the botox.
When the obicularis oculi muscle is flaccid, it bulges out, just like when you are really tired. If Botox is the source of your problem, it can take up to 8 weeks to resolve.
Swelling and edema are less likely and should resolve within a week.