Screen dims too quickly while using iPhone 3G

When using my phone the screens backlight dims way too fast, its only been doing this for a little while and it isnt a big deal because all I have to do is touch the screen again to make it light back up but it is getting really annoying... I thought it might be the sensors but when this first started happening I had been using my phone, uploaded some songs onto it and then it just started doing this. Before, the backlight would stay on for a good 10 seconds or so.. now it turns off after about 2.
iPhone 3G iPhone OS 3.1.2