Twitching Platy

I got 4 mickey mouse platys yesterday and they were swimming round nice together, then last night one started getting picked on by the rest. This morning she has spent most of it hiding in a corner twitching. Shes been out a few times and swam around with the group but shes still twitching.
Is she ill or stressed?

Water tests are all normal
20 gal uk tank
4 platys 1 male 3 female

Can you post water stats in ammonia,nitrite,nitrate,and ph, just check for parasites.

7.2 ph
ammonia - 0
nitrite - 0
nitrate - 20

noticed her rubbing herself on rock and gravel as well, this is a sign of ich isn't it?

Water quality is good so that not it, also it can mean temp is too low, yes rubbing is a sign of something wrong, is she doing it alot, check the gills to see if they look red and inflamed, or pale, or excess mucas on them.
Any signs of laboured breathing, does it look like something is irratating her skin.

shes been out a lot more this afternoon rubbing a little not a lot, gone up to surface a couple of times. heres a pic I took this morning, thought she was having a baby but it was just a hugh poo!! can't see any spots or irration on her but its my first set of fish so i'm not really sure what to look for and what behaviour is normal, don't think shes pregnant anyway can't see any black patch on her.
Thanks

I didn't see if you had checked her gills for signs of irritation, such as them being red, inflamed, excessive mucous, pale, or rapid breathing?

If the gills look fine, i would keep a close eye on the fish, watching out for parasites. If other fish begin to flash, or if this fish seems to be getting worse, I would consider the possiblity of parasites. I know sink flukes and gill flukes can be hard to spot some times.