Lump near anus in 5 month old

So my son has a hard round lump on the outside of his anus. It popped up within 3 hours. After a bowel movement. I called the on call nurse and she told me to schedule him an appointment. He goes in tomorrow. When we woke up this morning it was almost completely gone. He had a bowel movement  and it came back to being swollen. It looks just like this picture. I don't want to post actual pictures of my sons bum. I'm really worried. Anybody else have experience with this? It looks so similar to the picture that I'm almost positive it's a perirectal abscess...but of course I'm concerned it's more serious or cancerous :(