I cannot say with certainty what happened, as I have never experienced this issue myself, however I suspect that this has to do with some kind of malleability issue.

I would suggest that you wait until your wallet finishes syncing before you try to spend any of your bitcoin. It is possible that the electrum server you are connected to is running slow, (or has other issues), so you can change the server you connect to both by restarting electrum and by going to tools --> network, then deselecting auto-connect and selecting another server to connect to.

Once you have finished syncing, I would recommend that you not send any transactions until you have no 0/unconfirmed transactions associated with your wallet, or if you are using coin control, to not send any transactions that are 0/unconfirmed from the address(es) you are "sending from"