Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear that your girl is lethargic, lip licking, not eating well, has a sore back leg and thus seems to have trouble getting around and hasn't passed stool in 2 days.
If she has a sore rear leg she may have trouble getting into and staying in position to pass stools, so she may be holding her stool.
Dogs that aren't eating well won't have anything to make stools, but it may also be that she isn't eating because she hasn't passed stools and feels full. Do you believe her decreased appetite came before she wasn't passing stools or did she stop passing stools and then her appetite fell?
Lip licking indicates nausea.
Dogs with stomach upset and nausea may continue to eat treats and table food, but often refuse to eat their normal diet.
Simple arthritic pain may decrease their appetite a little, but I would not expect her to eat as poorly as she is.
She either has something more serious going on with her leg, and a bone tumor is possible given her large breed size and older age or she has something more, such as metabolic organ disease (kidney or liver disease) or a systemic infection like Lyme disease, or a mass in her abdomen.
Your girl needs to see a veterinarian for a thorough examination, complete blood count, biochemistry profile, possible radiographs of her leg and abdomen, and maybe testing for tick borne diseases depending upon exam findings.
In the meantime there are some things to try to get her feeling a bit better.
You can give her acid reducers to try and settle her stomach in case this is related to nausea. Either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle her stomach. These are quite safe and can be used for several days if necessary.
A couple hours after one of the acid reducers is given you can offer small amounts of water or ice cubes to lick.
If that goes well then late today or tomorrow start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 plain, boiled, white rice. Add 2-3 tablespoons of canned pumpkin (not pie filling, just pumpkin) to each meal to stimulate intestinal motility and keep stools soft and easy to pass. Give small meals several times a day. Feed the bland diet for several days, then start mixing in her regular diet and slowly convert her back over a period of 5 to 7 days.
Please let me know if you have any further questions.