Limping chicken

[Eggz]

For a week or so now one of my Orpingtons (called Java) has been limping, and this has been gradually getting worse. On further inspection we discovered that one foot has a lump on it and is quite badly swollen, and looks bruised. We hope to take her to the vet soon, but are struggling to get an appointment. Any ideas on what is wrong, or what we can do to help her would be great.

Sorry for the bad photo, she wouldn't say still for very long!