Tried to replace light switch but circuit breaker flipped with new switch, so put old one back and that flips the breaker too??
If the switch you are replacing is in an electrical box that is grounded there is a possibility that insulation in one if the hot wires is compromised and shorted to the box. This would create a direct short when the wire becomes energized and cause the breaker to trip.

The other possibility is that this switch controls an electrical outlet in addition to a light. Switched outlets are common in living rooms and some bedrooms. In bedrooms with a switched outlet a ceiling light may have been added after the original installation so that the switch controls both. In your case you may have replaced the switched outlet and wired it incorrectly. The error may not have become obvious until you came to work on and replace the switch.