Strange female platy behavior....

I was just wondering why my female platy is acting strange, if you all have any ideas. :)
She was living in my 55gal community tank, along with 2 other females and 3 males. I noticed that at least 2 of the females were pregnant, so I moved them over to a ten gal I have set up with cherry shrimp, just until they drop their fry. Well, 2 of the platies are swelling up,looks like they have swallowed marbles actually, but the 3rd is slim, actually getting skinny. She spends the whole day swimming to the outlet of the filter, (undergravel). Its kinda like a treadmill for her, I guess... periodically she takes breaks and goes to the floor to rest.
Well, I figured maybe she doesnt like the small tank, and shes not preggo anyway, so I put her back into the 55 gal today. Guess what she's doing? Swimming into the current of my over the tank filter. :/ The males are interested in her, but they cant seem to keep up with the current, and the one time one of them did, she nipped him, and continued swimming. I dont know how old she is, got her from my LFS, but they stink at restocking,so she could have been there for a while.
Anyway, she keeps losing weight, and Im worried that she will swim herself to death. Any ideas on why she is acting strange? The other females all act normal, she's the only wierdo... :) Thanks guys!!

Do you have water test results for the two tanks and temps for both? You say the other two females are very round. Look at them from above the tank. Do their bellies look like pinecones from scales sticking out? Was this female very round like that when you moved her over to the other tank? Is the female that's acting odd still eating?

No water tests, but the 10 gal is at 75, and the 55 gal is at 78 deg. Nope, no pine cone scales or any signs of disease... she eats like a pig, I guess from all the calories shes burning lol. None of the females were that round when I took them out, you could just barely tell they were pregnant, and this one, I never really was sure if she was or not. They have been in the 10gal for about a month and a half now, and I just put her back in the big tank today, incase she's doing it because she needed more space, but apparently that's not it, unless she's doing it out of habit...
I just fed them a while ago, she stopped to eat, and when the food was gone, she was back at it. Maybe I just need to feed her more....

Thanks!

Try to get test results(ammonia, nitrite, nitrate, alkalinity, hardness, pH). A lot of fish stores will do them for you if you bring in the water samples. How long have these tanks been set up? As far as feeding more, be careful not to overfeed. She may just be doing it because she likes it but the last thing you want to do is pollute your tank trying to discourage her. Maybe get a tank divider(the mesh type) and block her from the output and then see what she does. Are there plants or other decorations she can swim in and around? If not, then I suppose she could just be bored, too. Quite possible, too, that the males have been hounding her and she's discovered they can't keep up in the current(or at least that they aren't there whether she knows the actually logic behind it or not).

Thanks! :)
I know our local fish store does water testing, so maybe Ill check that out. Pretty sure the water's alright though, the shrimp are doing great....
She started doing it in the 10 gal, with no males, so I dont think its because of them. Yeah, both tanks have plants, decorations, etc, so she has stuff to do, places to hide, so I dont know... Maybe she's just crazy LOL.
Thanks again!!

Oh, sorry, and the 10gal shrimp tank has been up for about 3mos, (Im new to shrimp lol), and the 55gal has been up for a long time, 5-6yrs, I think....