hi guys. got a problem with my car. My car feels slower (less torque being felt) when i full throttle my car. Normally at low range i can feel the torque gradually building up until vtec kicks in but all the sudden, my car has gone sluggish and feels slower.

I've recently changed my car's Oil and when i drove my car, it felt faster again but after afew days later.. the car has gone weak and sluggish again

Any reason on whats causing this and how can i fix it.??