TLC is a live bacteria culture (no chemicals) so it is not causing you false readings. How much did you add (ounces)?

What types of fish do you have? Many fish will tolerate low levels of ammonia but corals do not. So I think the chain reaction Gary mentioned is correct. One thing died (either due to something with the water change or that could be coincidental) creating excess ammonia which caused other things to die releasing more ammonia etc. etc.

You need to control the ammonia levels though or you will begin to lose fish if it continues to rise. Make sure you don't have any dead livestock still in the tank that is continuing to drive the ammonia levels upward. You can lower them by water changes, polyfilters, more TLC but you need to keep them from spiking higher one way or another.