Cooked chicken in boiling water, bad?

So i buy in Iceland supermarket this package with sliced pieces of boneless cooked chicken, and its frozen. In the package it says cooke from frozen, but its already frozen, and it also says defrost in freezer overnight. I normally put the frozen chicken in a pot with water and boil the water for like 5-8... show more So i buy in Iceland supermarket this package with sliced pieces of boneless cooked chicken, and its frozen.
In the package it says cooke from frozen, but its already frozen, and it also says defrost in freezer overnight.

I normally put the frozen chicken in a pot with water and boil the water for like 5-8 minutes and the chicken its defrost and i eat it.

Do you think this is bad?

( i did this for a month and nothing happend to me, now i have sort of a gastrite, but i don't think its from that because i had it before a couple of times not to long ago).

Thks!
Update: Sorry it says defrost overnight in refrigerator.
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