Congo Leaning To One Side

The usual thing to worry about is head down or tail down swimming. Keep an eye on the congo for now see if gets worse.

I was quite alarmed when I saw my sundadanios listing to one side until I realised they all did it and the direction of the tilt depending on which side was facing the water flow - when they changed direction, they tilted the other way. I turned the filter flow down but they still do it.
Does your congo lean to the same side all the time or does it change sides?

As long as her scales are lying flat against her body when looked at from above, she could be either greedy or egg full. If it's the latter, she'll either lay them or reabsorb them.

Leaning both ways is not a bad sign. If it was only to one side and it got worse that wold give cause for concern.
If she is big, it could be just the way the water flow is catching her.