As my haiz comes I always feel that now I am free from the duty of namaaz. So what is this feeling. Is it a sign of low imaan?

No it's not a sign of weak Iman since it's just an Islamic responsibility that has been taken off. However in order to maintain connection with Allah, reciting dhikrs has been recommended. Therefore at the time of prayers, clean yourself and sit on the prayer mat and for the duration of the time it takes to normally pray, you can recite any dhikr you want. In this way you can maintain connection with Allah.