My AF is almost healed, but I discovered something behind my anus that feels like a hard pimple or lump under the skin. It's sensitive to pressing and it seems to be growing. Could it be the symptom of an abscess about to form?

BGD,
That's hard to say right now - Please just keep an eye and if it develops, go see a GP straight away. If it is an abscess, it will need proper medical attention and possibly draining.
Other than that it could be a hem or the formation of a Sentinel pile (especially if this is forming in the area where the fissure was), but these do tend to be soft and more fleshy like rather than hard. I wouldn't stress too much - just be vigilant about it. Fingers crossed, it's nothing to worry about for you.......Take Care.

I text my CRS and he said it could be an abscess. He is away, but recommended antibiotics till he comes back. I am scheduled for next Thursday.

Hi,
Yes, the anti-biotics will just be a precautionary measure....
Fingers crossed all goes OK on Thursday and don't worry too much, abscesses can be a bit painful when they inflame but this is generally short-lived and overall, they're not too difficult to treat......I also had one and once it burst, the pain reduced almost instantly. You may not have this though!