Chicken with infected knuckle??

[powersurbanfarm]

So I just noticed that my EE pullet(aprox. 8 months old) has an infection on the top of her feet where they bend. The left looks like an abcess, and the right one looks liket it had an abcess that has already popped. I got some epsom salt and soaked her feet this evening, and I also bought some tetracycline powder and mixed it with their drinking water. Does anyone have any idea what this is or how it happened? Is this the correct form of treatment?
Thank you,
Jen

Here are some pictures:

 

[seminolewind]

Is there a black spot on the underside of her feet?

 

[powersurbanfarm]

No, the bottom of both feet are completely fine. It just seems like an infection under the scales. There are 2 scales on her outer toe on her right foot that are s blackish red color too but they are not as nearly irritated.