Poorly Platy ?

It is always tricky knowing what is wrong with fish. Swimming funny is often a swim bladder problem and this can be due to anything from overeating to an infection or parasites. The fact that your platy is behaving normally otherwise suggests nothing serious.
She's not about to give birth is she? How much do you feed and does this fish grab more than her fair share?

In the absence of other symptoms, I would just monitor her for now. Things to look for - clamped fins, raged fins, not eating then getting emaciated.

Hi Cinders,

Sorry to hear about your platy I hope she soon perks up. Just a thought....how long have you had her? I only ask as you say she can't be pregnant as you have no males, but if you haven't had her long she could have arrived pregnant....I know, I know... that may sound a bit mad, but they're prolific little devils
Going from my experience of platies, they are ALWAYS hungry. I feed mine once a day and have a non feed day once or twice a week. My tank has masses of planting and they're often to be found having a nibble at it....just natural behaviour, I assume.

Anyway, someone will know for sure

Hi Resa,      I have had her about eight months, and she was pregnant when I got her, and had babies every month, but hasn't had any now for two months at least.   I know they can store sperm, but I think she has finished that now.    I think it might be an idea to withhold food for a day now and again, but she looks quite slim.  Thanks for taking an interest Resa, appreciated.

Thanks again Sue.