well, if my hindi serial viewing history had made me guess...then...
-there is someone in that household that he doesn't trust, (or something that Prem wants to find out) thats why he wants to continue acting a bit distant to Heer (and display that to someone in the house)
-he's mad at Heer for going out that day when he told her not to (yeah, but that would be weird)
-he's actually lost his memory and thinks he is Gaurav
-he thinks he is dying of a disease and doesn't want Heer to get too attached
-he's testing Heer to see if she recognizes him