Any time you find a lump it is always important to get checked by a doctor. The tests the doctor is doing are likely checking your blood for infection and other diseases. Included in these tests she likely included a test to check your blood iron. Low blood iron represents a slow bleed, typicle of colon and rectal cancer. The test is not difinitive, as hemorrhoids, ulcurs, and other diseases can also cause low blood iron. However this symptom is generally used to proceed with testing for these diseases.
If the lump is able to be inspected by you, then it can be inspected by your doctor as well. Your doctor will be able to check if it is simply hemorrhoids, inflamation, polyp, or mass. If it is a polyp or mass, the doctor may even be able to send you to a specialist to have it biopsied without needing a colonoscopy, since it is so close to the opening of the anus.
Your doctor is following the usual steps, so just work with them and they will make sure you are checked for everything it can be. Hope this helps.