If it is on the anterior wall of the lower rectum, you may be feeling the prostate. Posteriorly, you can actually palpate the tip of the coccyx. If you go way laterally, you can feel the ischial spines, but these should be on both sides.
However, except for the prostate, the other structures you would have to put some pressure on the tissues to reach them, to be able to palpate them.
It could be a thrombosed hemorrhoid. But, of course, the concern is always about a tumor.
So, for your piece of mind, you might want to see a colorectal surgeon for an evaluation.