Gecko not eating has some swelling on one side of mouth

Question
I am extremely concerned about my leopard gecko because she seems to be getting worse!  I have two female leopard geckos that have lived together in the same tank for four years now since I got them from Petco.  I have heat lamps, clean the tank regularly, feed them crickets and worms as well as the calcium supplements, the substrate is ground walnut shells.  (Let me know if you need more background info).  

Now the problem.  About a month ago one gecko began getting larger and the other was shrinking.  The one that has been getting skinny now has developed some type of swelling on one side of her mouth which at times almost prevent her from opening one eye fully.  I have tried to research online what to do.  So far I got a supplement that I have been feeding her with a feeding syringe daily to give her some  nutrients as well as water.  The only thing I can find online that sounds similar to the problem is mouth rot but I am so confused how one gecko looks awesome and is extremely active and the other is declining.  Plus the tank is not dirty!

Please any information or advice you can give me would be extremely helpful!

Thank You!

Answer
Hello Amy,

Do you have any pictures of your girl I could see?
How old is she?  
That sounds like it is going to be a mouth abscess or as you say mouth rot.  She may have had a feeder bite her, or her immune system could be suppressed.  
Do you think that she could have ingested some of the walnut shell substrate?
My recommendations are to remove the loose substrate as it is almost always an impaction risk for geckos as they like to lick everything.  You can use paper towels, non adhesive shelf liner, tiles, or colored felt is always nice, too.
Are you using an undertank heater too?    
Continue to use the syringe feeding method.  What foods are you giving to her?  The chicken or turkey baby foods are usually tolerated the best.
What are the temperatures in the tank?   
Is the other gecko possibly dominating her now?  Sometimes that can happen, even if they appear to get along.  You might want to consider separating them for awhile until she recovers & is eating on her own again.
To help with the mouth area, get some betadine & dilute it down until it resembles weak tea.  Then use a swab & dip it into the solution then clean out the mouth as best you can.  Try to do this a couple of times a day to help with any infection.  Does it appear that it is getting infected?

Let me know how she is doing.
Tracie