Hi Mr Verma,
Chronic subdural haematoma
means that there is a collection of blood in the space between the lining of the brain. This happens in elderly people whose brain shrinks in volume, but the skull volume remains the same, so there is stretching of the veins which ooze. Since your dad was on Warfarin
which causes thinning of the blood, the bleeding tendency is increased. the drowsiness
could either mean increase in the pressure on the brain by the bleeding or a side effect of the medication. Since your dad is still having headache, it would be very advisable if you could consult a Neurosurgeon to see if evacuating the collection by putting some holes in the brain (burr-holes) at the earliest.
Please do let me know if you have any other queries