Did I lost the chance to get my BCH from Aug 2017 fork? I had some BTC before the BCH fork in my wallet, but I didn't know I could claim the same quantity on BCH too, I've just read about how "hard fork" works :(

Then I sell my BTC, that means now I cannot get my BCH anymore? or the BCH should be there even after selling my BTC?
Your BCH will still be at the same address your BTC were in as of the fork date. BCH is a separate chain, so if you send BTC transactions it will not affect the BCH balance. 

In order to move the BCH, you will need to install some BCH wallet software, and then import the private keys/seed for the addresses that were funded at the time of the fork. You can obtain the keys/seed from your BTC wallet, but be warned: it is prudent to empty out your BTC wallet before doing this, in case the BCH software is malicious and attempts to steal your funds. (even if it is a small chance, better to be safe) 

The BCH software will be able to communicate with the BCH network, allowing you to access those coins.