Mollies behaving weirdly

Feb 19, 2018
Hi I have a 120litre tank heated to 25, with a fancy goldfish, one golden Rasbora, 5 red rainbow fish, 3 orange mollies and 3 Dalmatian mollies in.

Just recently I've noticed two of my damaltion mollies behave weirdly. The male seems very 'bendy' sometimes when it's swimming. Also one of the females does that too (though not as frequent as the male) and today I saw her hanging upside down for a few minutes until I went over to the tank, whereupon she started swimming with the others again.
Does anyone have any ideas what is going on? I checked the ammonia, nitrate and nitrite levels and it was still in the 'ok' zone according to the colour chart. However I did still do 30% water change as well. My other fish all seem fine.
The female has quite a fat tummy. Before now she has looked like that when she was heavily pregnant. Is it possible that she could 'hang' from being pregnant?