Well, can't rule-out anal sacs, but certainly doesn't sound like them if you can actually "see" the bump and because of positioning.
I am a little more worried about an early mass forming, or perhaps, some type of hypersensitivity reaction.
Now, either way, if there is any amount of scooting, then those anal glands must be checked, otherwise, the scooting will exacerbate the inflammation involving whatever this bump may be. It could even be a bite/sting from an insect.
However, we do see a lot of benign and not so benign growths involving the rectal tissue, so it is worth getting this examined sooner than later when it is smaller and easier to address.
Depending upon examination, certain topical medications could be recommended, an even a fine-needle aspirate test.
A fine-needle aspirate test is performed usually without anesthesia and is very quick. Your veterinarian takes a few needle samples from the lump (the needle is no larger than the needle used to give vaccines). The sample is smeared on to microscope slides and submitted to the laboratory. It is not always as definitive as a biopsy (this is a full tissue sample), but often it can yield enough information to determine if it is anything to be worried.