sick rat with dripping foamy saliva

Question
I have 2 rats, Kala and Venus.  They're sisters, about 6-7 months old.  Venus is usually very playful and excited to run around when I take her out... but tonight she all of a sudden became very languid, lying around, and then I saw that she was dripping a foamy saliva from her mouth.  At first I thought it was diarrhea, b/c i know that rats don't vomit.  But it WAS coming from her mouth - regurgitating something.... I was thinking that maybe I overfed them - is that possible? I gave them a few pieces of potato, and also some sweet potato.  Then later in the evening I gave them 2 very small french fries.  They love those... usually I give them in moderation, but maybe all of that was too much potato??? is that what's made her sick now? Kala is fine though.  It's only Venus that's sick.  

If I take her out of the cage, she'll walk around slowly (not like her usual curious, running around character).  When I put her in her hammock, she'll just lay there and roll around with her nose over the edge.  That didn't look healthy, so I took away the hammock.  She's now lying in the bottom of the cage, in a dark corner under a few extra rags that I put in there for her.  Usually both Kala and Venus will lie down on the top shelf under several rags that I have there, eyes closed. She's not as warm as she usually is either. She's breathing, but it's a little laboured.  She is just totally acting out of character :(  

Kala keeps licking her off, all the saliva/vomit/whatever it is.  She's sometimes a little rough with her, walking over her and licking her all over - should I separate them? but they've always been together and I'd feel bad to separate them.   Maybe she's helping her?

What can I do for Venus? is there anything I can do? or is the vet my only option? Should I separate them?

Thank you for your time.  I look forward to hearing from you.

Answer
I am hoping that she is OK now....let me know.  Sounds like she had something caught in her throat but she was still able to breathe.  You may notice they drag their face on the bottom of the cage, drool, have a foamy substance coming from their mouth, and want to be left alone, which you should, as long as the rat is able to breathe ok and shows no signs of distress.  My boy just did this a few weeks ago....I hated every single moment of it but after just an hour he was back to normal. This goes unnoticed with many rat owners since we dont watch them 24 hrs a day.  Let me know if she is back to normal today.