ruchika,
First of all Don't be panic, believe in yourself and trust in Almighty. You have reached to this point and im sure u will clear this interview.
Honestly, this is first such instance where interview is scheduled after Medical and rprf fee.Very difficult to explain why this interview being schedule
Did you changed your job before medicals?
Did your ecas status shows "Medical Received" line?
When did you notified about interview and when it is scheduled?
Seniors your suggestions are highly appreciated, please guide her.
Good Luck,
Yameen