Walaikum salam, Brother.
I would assume this is nothing but Satan whispering to you as you pray. He is trying to distract you, and unfortunately, it looks like it may be working. I am not a scholar, but I would assume that this would not be counted against you as Shirk, since you are not having these thoughts on purpose... and even then, your distress over this situation is a good sign of your solid belief in Islam.
And Guys pls tell what to imagine while Sujood if you have any answer..
Personally, I don't "imagine" anything as I make sujood, I simply focus on my worship, and that is what I recommend you do too. Try your hardest not to let your mind wander and entirely focus on praying. I don't struggle with haram thoughts when I pray, but I still sometimes find it hard to focus, so it's important to make sure you don't think about anything else outside of your prayer.
May Allah help you overcome this.