i'm still able to send 'old' TRC coins, and see them included in later blocks.
Any chance you can extract the full reject reason from your log file ? (the client log, if you were solo-mining) ; could be some time-sync issue, or your local client/daemin just "saw" the new block too late.
(looks like cgminer output)
There is no reason why a 51% attack needs to affect txns. It can just monopolize mining and have no other impact on end-users. (i.e. unapproved miners get their blocks rejected. other than that everything is normal)