Probable dx: External haemorrhoids. Severe constipation leads to increase in abdominal pressure leading to haemorrhoids, hernias, fissures etc. Since you hv been experiencing burning sensation i.e. pain while defecation. The haemorrhoid is present below dentate line.
Next Steps
Include fibres like Isabghol husk, green vegetables, fruits in diet. Drink at-least 2 L H2O.
Health Tips
Use laxative SOS and an ointment w composition Lidocaine + A/b.