ok that's real common with interdigital cysts . In between the fingers of the paws you notice some abscesses that grow in size when liquid or pus accumulates. This is probably due to an infection that has access to the tissues under the skin. Before the abscesses appear, the dog insistently licks his paw between the fingers. A short while later, he starts to limp due to the pain he feels, and as the tumefaction increases, he won't let you examine his paw. After a few days the abscesses pop, releasing pus and blood, thus alleviating the pain. As long as there are no complications, these heal normally and the animal recuperates completely.
It almost looks like there is something in there. You could try soaking it in an espom salt mixture to see if it opens up. In some dogs, interdigital cysts are an ongoing and recurrent problems. My last dog had issues with them recurring often. I'm dealing with one on Derby right now. His is almost healed up but he required antibiotics for this one. Even though the cyst opened and drained a little bit, it still caused him immense pain and he limped for about 5 days.
If you want to soak the foot, I found that using a gallon-sized ziploc baggie filled about 1/4 full with the epsom salt solution works well. When the dog is laying down, take the paw and put in in the corner of the bag and use your hand to hold the bag closed around the foot/leg. Keep petting them and talking to them. Derby was pretty good about it. I soaked his foot for about 10 minutes. Have some towels hand, place one under the foot being soaked. You can tip the bag so that the part that needs to be soaked remains under water.
I had thought about soaking her foot in some espom salt. Of course I could see me trying to get her to stand with her foot in a bucket with espom salt and water ... you know, like we use to do with the horses. I will go try the baggie. Thanks everyone.