What's Going On With My Red Platy?

Please can someone advise me what is going on with my Platy fish. It is a female and I was almost certain she was pregnant because she was very large, until she started doing these thick white poos. She is very active and eating very well, so I just can't understand what's causing these poos.
 
I am uploading a few images so you can see what I mean.

What do you feed her? Any new tank additions, plants, decor, or fish? 

Thanks for your reply. I feed her King British Tropical fish flakes in the morning and crushed and shelled green peas in the evening. This is what I feed my other fish in the same tank, and they seem to do well (though my Angelfish seem to dislike the peas.)
 
The tank mates are:
 
1 Other Red Platy
4 Yellow Sunset Platy
2 Angelfish
2 Kribies
 
Ten fish in total. Its a 130 Litre tank. The yellow sunset Platies were put in the tank at a different time than the others (later on) but are from the same aquatics store.
 
Thanks for your help
Also.. I used a filter and ornaments/plants from a previously cycled tank to keep the good bacteria for this new aquarium. I am worried that it also contained bad bacteria? The previous tank wasn't mine... I bought it on Gumtree.

*UPDATE* - She is pooping these thick white things very frequently, maybe 3/4 times every 30 minutes.

Can you remove these thick white things from the tank? do they look like a worm or any sorts? She may of had a worm infestation and the pea's being a laxative are flushing them out of her system (which is actually good)

I have given the gravel a good hoover today in order to get rid of faeces and debris, but there's brown particles everywhere now! It has settled like dust on everything but all the fish in the tank seem to be crazy for it pecking at everything it's settled on. What's is it?!

The platy is still active and eating loads. I have fasted all the fish today and done a partial water change.