Diarrhea is much more common in dogs than constipation.  Signs of diarrhea mimic constipation.  Dogs feel the urge to defecate but there is no stool present in the colon--this makes them strain with no stool produced.  Feed boiled chicken and boiled rice to your dog for 2-3 days.  Most cases of diarrhea are mild and self-limiting.  Seek veterinary medical attention if signs last longer than a few days or any other signs develop.

The best thing to do is make sure she gets plenty of exercise and has a good diet.  Don't feed her any table scraps or treats that aren't specially produced for dogs.  Constipation in dogs is almost always due to 'extras' in their diet.  High quality dog foods contain the right balance of fibre and nutrients to keep your dog healthy.  Also make sure she is drinking plenty of water especially during summer months.