How to tell amperage of a circuit breaker in basement? Also... adding a dedicated line for Treadmill?
Since everything is going out the circuit is overloaded. Standard breakers are inverse time trip this means if right at the max level usually printed on the handle (15 or 20) amps the circuit may hold for 10 minutes before tripping but if a short were to happen it would trip immediately. If you turn a light or 2 off you may be able to use the treadmill. Other things like freezes on the same circuit can cause the circuit to trip quicker if running at the same time as the treadmill. It is possible your old home had a 20 amp circuit and or was lightly loaded so no problem, now if a 15 amp circuit and  a heavy load the breaker is doing its job of protecting the wiring.  A new branch circuit would be a good path forward and possibly having the garage on its own circuit for the time you may want to run a power tool the same thing will happen there.