|I've never done flash fiction before but I imagine it's hard. I want to point out a few things that are easily fixed. For instance, below there is no indication as to where the screeching is comming from.
"Hello" Nikhil knocked on the half closed door. The only reply was the screeching noise.
One way to fix this is to say that the door opens a bit.
Here a "the" should go after "got."
Nikhil's curiosity got better of him and he walked in,
With this little blip, "step" should be "steps." Honestly though just omit "of is." It flows better.
The wooden floor creaked with each of his step.
Concerning this paragraph, first omit "chillingly." It is superfluous. Next, drop the "to" after "the white curtains. Following this, you spell Nikhil's name differently. Finally, the second sentence is garbled. Following the comma of the second sentence, perhaps you could have ",that made Nikhil wonder why he thought to enter after the door behind him slammed shut."
Nikhil felt a chillingly cold breeze blowing in from the open door, making the white curtains to flutter. The air outside was absolutely still and hot, making Nikil wonder should he have come in when the door behind him slammed shut.
This paragraph is the last correction I will make. First, omit the first comma and put "as" between "startled" and "his." To put it in simple terms, the second sentence is stilted. How can Nikhil get up when he is still stuck in something? finally I will point out that what you have after the last comma is understood. By this time it is obvious he knows he is stuck.
Nikhil Stumbled back startled, his leg got stuck in something making him fall with a loud crash. Nikhil got up hurriedly, wanting to get out of the house, but he realized his leg was stuck in a hole on the wooden floor. .
There are problems with the dialog, but a quick re-read will expose them. I think a good look over will greatly improve your work. Happy WdC anniversary.