First reason can be ----
when they had a accident ,nupur was badly injurd or may be she was pregnant by that time but did not know.she was injured badly n went in to coma n shifted to different hospital and doctor mixed up both nupurs body n gave wrong information to Mayan n gunjan and told them nupur is dead. Mayank n gunjan believed whatever doctor said n did not tried to find nupur. After few months nupur came out of coma n asked doctors abt her family,doctor might hv told them her whole family died n also she lost her child, she was shatered to hear this news n could not control herself n she had to go to trauma centre and she met nurse who looked after nupur, may be she adopted nupur.once nupur got better n one day she came to excel college to colect her certificate n saw mayank n riya together happily n saw gunjan n samrat also.the felt abandoned by her own family n decide to stay away from them.may be she has seen mayank ,samrat n gunjan at dhanno party .she realised that nobody want her so she decide to lead new life on her own. I think when she will meet mayank , amnesia thing might come and she will pretend that she lost her memory to avoid mayank.nupur hassuffered more than anyone and felt betrayed by her own family .....
Second reason Can be -------------------
may be nupur was pregnanet but she was not aware n once she regain she asked doctor abt their family and doctor may be told her that no body came to see her n doctor also told her that she lost her child n due to this accident she will never get pregnant again.nupur knows mayank wanted child after 3 yrs but nupur cannot fulfill that dream.she is also shattered that nobody came to see her. But after 3 yrs once she got better she went to excel to collect her certificate n saw Mayan , gunjan n samrat leading their life happily .she felt abandoned by her own family n decided to stay away from them. . Both reasons r valid to some extent n only one person who suffered more was nupur bhushan sharma.
This is my pov n i know doctor said report is clear before they were leaving for America .. but that does not they did pregnancy test as well đ
Please leave comment if u think reasons r valid n also tell me which reason is more closure to nupur condition. Thanks for taking time to read