Hold down the sleep button for an extended period of time until it says turn off. Slide your finger across to turn off. Press the sleep button again to turn it on. That is a soft reset. To hard reset (which is sometimes used when things are really messed up on the phone) press and hold the home button (the circular one right below the screen) and the sleep button together for about 40 seconds.
The only thing I can think of is that you might of had too many things running at once. Did you have a lot of websites on safari open? Sometimes when safari has a few websites up it will continue to run those, try holding down the home button for about 20 seconds when in safari, it should close them all down.
It sounds to me like you're never completely shutting your phone down. Many people do the same thing and only put the phone to sleep after pressing the lock button. You need to hold down the lock button until you get the "slide to power off". Shut it down and then restart it and you should see better performance.