Should I get a battery replacement?

I've had my iPhone 4 for 6 months and my battery shouldn't be this drained yet. I can't even look at my phone without the battery percentage dropping. Even when im not using it it drops. Sometimes I even have to charge it twice a day. When surfing the web or using apps it goes from 100% to 90% in 10 mimutes after being fully charged. Is this normal or should I get a replacement?