I would be shocked if it was an infection. See if the pain persists and worsens and whether you have other symptoms of infection (that definitely aren't withdrawal symptoms), and if so, go to a doctor. It is NOT a good idea to be randomly taking antibiotics. Antibiotics are really bad for your health and kill your immune system, so they should only be taken when you absolutely unequivocally need them. Plus what makes you think these particular antibiotics would treat it even if it was endocarditis or something?
If you hadn't already said you had a bowel movement today I would be 99% sure it was constipation. However, it could definitely still be constipation (just because you went doesn't mean there isn't more in there) or a host of other bowel/colon related issues, which are extremely common for opiate users. Most of the time pain in that area is caused by your colon.
If you're really that worried or it's extremely severe and getting rapidly worse, go see a doctor.
More specifics would be helpful in giving a more accurate assessment. The exact area of the pain, what it feels like, what your other symptoms are, whether they go away completely when you use, does it feel worse when you press on it, does it feel better or worse when you massage the area, etc etc.