new plant only about 2 weeks old is turning yellow and brown fast. help

i have a 2 week old plant that was 100% green and looking healthy 2 days ago now today 2 of the leaves have turned yellow and some brown too. any help? i havent given it any nutrients yet either. when do i need to transplant. it is in a solo plastic cup 12 or 14 oz size (cant quite remember). thanks you
 
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