How the blockchain knows that its been tampered with? I know that the transactions stay in the block and hashed alongside few other things and that hash is stored in the next block. And I understand that if you try to tamper with one of the transactions, the hash will change and it will affect all the blocks that come after it.

However, how does a block know that its transactions have tampered with in the first place? 

Assume that block #1000 has 10 transactions.
Now, the hash has been calculated and stored in block #1001

so, if I go back to block #1000 and add one more transaction, this will change the hash and it wont be the same one stored in #1001. 
However, how the hash of block #1000 changes in the first place if it was already calculated? does this mean that the hash of each block is recalculated regularly?
Hashes don't actually 'change'. When you add a transaction to a block you are creating another block.

It's like an author published a book and then next year printed a new edition with an extra comma in the page 36 and the fans said he released a 'new' book. It doesn't make sense for books but for blockchain every bit counts.