Could it be anal cancer??

My dog has gone to the vet several times, and they haven't determined what's wrong with him. He has trouble defecating. He cries and has a very hard time every time he tries to go. He used to get excited when it was time to go outside. Now he runs to his bed because he does not want to go. The other day he had a yellowish/orange liquid come out. Its was a large amount for a 5 pound dog. (I have been giving him sweet potato's so Im nit sure if that is related). He went to the vet and they cleaned out his anal glades and a month later they were full again.
From my personal research all of this points to anal glade cancer. I would really like an experts opinion before he returns to the vet this Wednesday.
Regards,
Carrie

7 year old male Chihuahua