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1.The history and anatomical location suggest the possible cause could be hair follicular infection with collection within, next possibility is sebaceous collection due to ductal blockage.
2.Other possible cause since you have mentioned about the
constipation - if its arising from the anus / anal region the possibilities of haemorrhoids can be think of.
3.In any way i would recommend to not squeeze or scratch the area as it will complicate, maintain local hygiene,clean area with antiseptic liquid and soft clothing which do no cause any rubbing effect on the lump.
4.How ever it needs to be examined - if infective nature needs a course of antibiotics, in sebaceous nature needs monitoring, haemorrhoids needs finer diet, more of water will help ( avoid junk and spice, fat foods).
5.SO i would suggest get an appointment and get examined and treated accordingly. Good luck.
Hope i have answered your query,
any thing to ask do not hesitate to ask.
http://doctor.healthcaremagic.com/doctors/dr-ganesh/62888