It could very well be a parasite infection, one of the most common being tapeworm, but there are quite a few others too. Pet store meds don't treat all worms and even the ones they do treat, they rarely treat all stages - so by the time you're done treating, it's cost you even more than the vet trip would have.
This could also be a symptom of Anal Sac Infection or Impaction. In most cases a veterinarian can express the glands and provide quick relief for your pet while teaching you how to do this rather unpleasant, but sometimes necessary procedure. An antibiotic may be recommended as well. If this is done, make sure you finish the course exactly as directed. Stopping early will only end up costing more when you have to start all over again.
There's more info and photos about what might be happening here: www.dr-dan.com/analsac.htm and even more here http://lowchensaustralia.com/health/analsac.htm
This should resolve pretty soon either way - once your companion is seen.
Good luck and I really hope this works out for you and yours
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Animal Rescue&Care Org Owner; Animal Care author; Animal Behavior Consults.;Cert. Avian Specialist;