Small hard pea size lump in the beginning of my anus (inside)!!!! It's under the skin! Please tell me what is it? I have no STIs

Hemorrhoids? A hard lump under the skin can sometimes be a hemorrhoid (swollen or clotted rectal vein). Depending on the location they can be painful or not painful. If painful, any general practitioner can remove a clot from a hemorrhoid with immediate relief. Otherwise, keep an eye on it and visit your doctor to confirm the diagnosis if it doesn't improve.