i'm getting more power? but i didn't do anything? Options

[azian_advanced]

need some clarification:

to start off, i bought my car with 97,000 kms on it, and the engine was pulling alright w/o problems what so ever.. but now, i notice the more i drive it the stronger it becomes... i got the car nov 2005 and last summer of 2006 i noticed that it felt a little stronger than when i first got it. and recently, last sunday, i was on the hwy going 160 kms/hr (a little over 100 miles/hr) and it was probably my 3rd time ever going over 3500 rpms but never passed 4k. and at that speed i was actually nearing redline, then i reduced speed again.. did that about 2 to 3 times (i was not racing btw! i don't race. just to get the point out. i'd say it was more like a 'spirited run').

later i stopped at a coffee stop for donuts for a little while. when i got back to drive the car again, it seemed like i was getting more power for some reason. it was like i was getting another 10HP or so i think. the car would pull stronger and the rpm would rise quicker. it's been like that since last sunday. i dunno, but did that 'spirited-run' have anything to do with my engine getting more power? my car is now at 109,000 kms on the odometer. since the time i bought the car, i think there has been an increase of about ~15-20HP.

btw, nothing was changed in the engine or drivetrain. and i have no engine or performance mods at all (just suspension), so everything else is still stock. i've only resetted the ECU and performed regular oil changes . can anybody explain what's going on?