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It could be an anal gland problem as this is common in dogs and if the lump is on either side of the anus it would mean that the anal glands need to be expressed. If caught early they can express the glands and provide antibiotics and anti-inflammatory medication. If not treated early then in some cases they are not able to express the glands and they have to be lanced and have a drainage tube put in which can take longer to heal and be more painful to your dog. If you have any further questions please let me know.