There are conditions that have similar symptoms. But for POIS, the trigger is specific, it happens after most, if not all, ejaculation or orgasm.
On the forum, it has been discussed that conditions like chronic fatigue syndrome, Lyme disease, Long Covid Syndrome, etc... have similar symptoms. But those tend to be chronic, whereas POIS will last for a few days to a week or so, and symptoms subside if no other ejaculation ( although in some cases, just pre-cum or sexual arousal will elicit some POIS symptoms in some members).
It may get complicated if someone has a chronic condition, and has POIS in addition to it. The chronic symptoms are always there, but get worse during a POIS attack, and some other symptoms appear that are not there outside of POIS. For example, you can have chronic anxiety, but it gets worse during a POIS attack, and other symptoms may add, like brain fog.
So, many presentations are possible.