Suggest Medication For Painful Pimple On Anus

i can feel a pimple or something of the sort just in the begining of my anus. Few days ago when i used toilet paper a noticed very slight blood. no more after that. the pimple only hurts when i touch or tighten the muscles in that area. no pain at any other time. i was very constipated just a couple of days before i felt this pimple. what should i do? is it something i should worry about?