Try a couple of things. In settings, go to the Application Manager, tap the menu button, and tap Reset app preferences.
Then shut the phone off (completely, not just the screen) and boot into recovery mode: (press and hold, simultaneously, the volume UP, home and power buttons. As soon as you see the Samsung logo, release all three buttons.)
You should see a menu. Use the vol up & down buttons to select clear cache, then press the power button. Then choose the option to reboot your phone (or maybe it's exit recovery--I don't remember exactly), and reboot the phone. See if things are better now.
If not, the only other thing I can suggest is to do a factory reset.