A strange "bump" on Owen

DISCLAIMERS: I am NOT a veterinarian (I am a veterinary student, but have not finished school yet) so do not take my advice as that of a veterinarian! As always, consult your veterinarian for any concerns you may have about your pet(s).
Also, there are photos below of a skin lesion that might be gross to some people so read below if that's OK with you. 

Around the first of the year I bathed Owen (he gets regular baths) and, when I did, I noticed a bump on his left shoulder that had not been there before. It didn't seem to be bothering him, but was about the size of a small pea. I was somewhat concerned at that time but thought that maybe he had scratched himself or something so waited on it to heal.
I continued to watch the spot and, when it didn't go away by the 16th of January, I decided to take some pictures of it to document what it looked like and how large it was so I could monitor it more easily since at that point, I didn't think it was a wound. By this date, it was slightly larger (the size of a small button). Here is what it looked like on that day (16 of January).

January 16 2019 "Bump" on Owen

January 16 2019 "bump" on Owen

At that point, I decided to start trying to figure out what this bump was (being a veterinary student, I had access to good information to do so). I fairly easily discovered that it was likely either a mast cell tumor (VERY concerning as they are malignant) or a histiocytoma (MUCH less concerning). To learn more about histiocytomas, go here. Given Owen's age (3, almost 4 years old), and the location (front limb), I was leaning towards it being a histiocytoma (a benign skin tumor) but wanted to document size and characteristics just to be sure. I decided to wait to take him to the vet (knowing that he was going to be going in March for his annual heartworm test anyway so if it didn't go away, I could get it seen about at that time), monitor, and see if it went away since histiocytomas usually go away within 3 months.
Here's what it looked like on the 28th of January.

January 28th "bump" on Owen

January 28th "bump" on Owen

At this point, I was fairly convinced that it was indeed a histiocytoma as it wasn't growing rapidly like mast cell tumors often do, nor was it extremely itchy for Owen (it was maybe slightly itchy but not like I've seen dogs with mast cell tumors deal with). I continued to watch it and around February 12th, noticed that it was scabbed over and felt smaller. Here's what it looked like on that date.

February 12 "bump" on Owen

Between this time and now, it scabbed over a couple of times, each time getting smaller and smaller as the scabs peeled off. Now, the hair is starting to grow back in and the area is flat again. Here's what it looks like today, March 12th.

March 12 "bump" is gone!

So, given the progress and timeline of this "bump", I believe that it was a histiocytoma. Can I prove it? No, because I didn't subject Owen to a biopsy of the lesion. However, this is the most likely condition to behave like this and the signalment fits (Owen is young, the lesion was on his forelimb, it came up rapidly, it regressed within 3 months). The cause of histiocytomas is not known. However, it is thought to be induced by an immune reaction of some kind. Owen already has allergies so maybe that made him more prone to developing it. Who knows. The good thing is that it is gone and, hopefully, won't come back!