I installed electrum bitcoin diamond 3.1.2
I found my seed from 12 words.
Did you move your BTC out of this wallet before compromising the seed by entering it into a BCD wallet ?
I checked the bcd I found and sent it to the exchange.
But the BCD was not sent, but the BTC was sent.
Did you check the block explorer for BTC and BCD ?
Were the BTC sent to the address you provided ? Or to another (random) address? You might have downloaded malware which stole your BTC.
I think I found both BTC and BCD from 12 words.
This makes sense, since BCD is a fork of BTC and your mnemonic seed phrase does reproduce the private keys for the addresses you 'received' your forked coins to.
I am sure I sent a BCD but it was sent to BTC.
So, did you send BCD or BTC ?
First you have told that the BTC were sent. Now you are telling that your BCD was sent to an BTC (address) ? Please clarify.
The exchange had the same BTC address and BCD address.
This is quite unusual since exchanges create a new deposit address for each coin/customer.
What exchange were you using?
Can a BTC be sent from a BCD wallet?
If the BCD wallet is buggy or malicious, yes of course.
If it is proper coded and trustworthy (which you could never say about BCD), no.
Can't I find my BCD in this case?
What should I do if I can find it?
Please clarify your whole situation. There are some contradictory statements inside your story.
We will be able to help you once we understand you unambiguously.