When I click on a block, it automatically replaces the item in hand HELP!

[Imagineer88]

So, I'm not sure if this is a feature or a bug, but whenever I left click (attempt to build) on something, the game automatically replaces the item I have (or adds to the hotbar) in my hand with the block that I clicked on. This makes it impossible to build anything. For example, when I click on a villager, to trade, I get a Spawn Villager egg and I cannot trade with him. Please help!!!

Edited February 27, 2018 by Imagineer88
added the for example

[Imagineer88]

Nevermind, I figured it out. My pick block key was the same as my place block key.