"Calamity" in FFXIV is not a general term. The other, more specific term for it is a Rejoining. There's a very good reason the Voice says "Twin Dooms" are coming and then mentions a Calamity and then a Flood. They aren't interchangeable. We also have yet to see any evidence of a Calamity happening on a Shard and a Flood happening on the Source. We do have lots of evidence of Calamities happening on the Source and two Floods happening on the Shards. Going by Occam's Razor, the Source is gearing up for a Calamity again.

A Food and a Rejoining are opposite ideas in a lot of ways. A Rejoining is when a Shard joins the Source because the walls between the Shard and the Source have been broken. A Flood is when a Shard receives an influx of Light or Dark so strong, that either the Shard looses all aether (Flood of Darkness) or the Shard looses all life (Flood of Light). Most importantly, a Flooded Shard cannot be Rejoined to the Source. I'd be very, very surprised if the two phenomena were capable of happening in the same dimension at once. I would not be surprised if the Shard that is about to be Rejoined to the Source in the next Calamity is about to go through a Flood and everyone (Hydaelyn and the Ascians) want us to take care of that somehow...