Bee stings are common with dogs, but just like humans some dogs can be allergic.
Using a credit card, scrape out the part of the stinger you can see or pull the whole stinger out with tweezers, but only if you can reach it. Breaking it will allow more poison into your dog's blood stream. If you can't reach it, leave it alone it will work its way out or your vet can remove it.
Clean the area of the sting with a baking soda and water paste. This will also help if the area starts to swell or gets itchy.
If she is having problems breathing, acts weak, disoriented, or is vomiting or having diarrhea, see a veterinarian immediately. If this is not a issue you can keep a eye on her for the next 24 hours. Most dogs are OK, but the fact is some dogs are allergic. If she has any of the above symptoms she will need to see the vet. Since she just had a walk, try to keep her calm for the next 30 minutes. If she is still having problems or the breathing gets heavier then just get her to your nearest emergency vets office.