A hard, painful lump right over the tailbone for 15 days in a young desk-job patient is almost always a pilonidal sinus or abscess. Long sitting hours push hair and debris into the skin there and it gets infected. This will not disappear with ointments - if it is already firm and tender, pus is likely collecting inside and it needs to be examined and drained rather than waited out.
Next Steps
See a general surgeon in person in the next day or two for examination and a soft tissue ultrasound of the area - if there is a collection, drainage gives immediate relief and Linezolid alone will not fix it. Meanwhile sitz baths with warm water twice a day, keep the area dry and hair-free, avoid sitting continuously, and take paracetamol for pain. Book a Practo consult with me if you want me to review the antibiotic choice and your low counts alongside.
Health Tips
Fever with chills, spreading redness or foul discharge means go to emergency the same day.