Have been latching my lo ( 3 week plus) for two and half hour already.. lo will fell aslp on my nipple but when I bring her to the cot she will Wake up and cry and stick out her tongue and appear hungry.. what's the reason behind it

Seems like baby associated feeding and sleep together. Baby is comfort feeding, thats how baby self soothe to sleep. So when you put baby down, naturally baby wakes up as his comfort is gone. It is very common. Baby is 3 weeks plus still very tiny. What you can try to do is unlatch and carry baby. See if it works.