If I have made a BTC transaction after the fork and haven't claimed my BCH before, have I lost them? The title sums it up pretty well, I made a transaction with bitcoin core wallet after the fork, but I hadn't claimed those bitcoin cash, can I claim them, or have I lost them?

Thanks!
Bitcoin Cash is an altcoin; things that you do on Bitcoin Cash will not effect your Bitcoin and vice versa.

With Bitcoin Core, you can install it's Bitcoin Cash analogue, Bitcoin ABC. From there you import your Bitcoin Core wallet into Bitcoin ABC by copying it to the Bitcoin ABC datadir. Then you can spend your Bitcoin Cash normally.