if you don't have a lot of space left (as you can't update, I assume this is the case) you are potentially making the phone work more if you are on it alot. Might be time to consider deleting some apps/photos/videos etc to make more space. Other things could be a new app you have added recently is running in the background and using location information (Facebook and google search are 2 of the worst offenders off battery life). Other things could just be that the battery is old or going bad. If you have a 5 check and see if it is one of the recalled batteries.

Also, ios8 will tell you what apps or services are sucking battery life. So consider updating -plug into itunes and you don't have to delete the apps. Also, back it up to icloud or itunes before upgrading, just in case.