could be LOTS of nasty things; cutting/opening it would not be top of my list for home remedies if you don't know what it is; if malignant, you could really be in trouble;
if anywhere near area where you routinely inject vaccines could possibly be a fibrosarcoma from the adjuvant in the vaccine ( more common in cats, but certainly can happen in dogs ); could be a bening lymphoma, but you don't want to mess with it yourself;
if it gets 'ugly' or she starts to worry with it - rubbing it, scratching at it, etc., you really need to get it looked at, especially if you see any rapid growth;
Good luck -
Kathryn