iPhone losing charge?

I have no idea what is happening, but for the past 2 days my iPhone has been loosing its charge way too quickly. Usually I can let it sit for two days before having to charge it. Yesterday, it died on me twice. I fulled charged it the night before. When it died the first time, I had to recharge it. Shortly after that I go to see what time it is and I get the no battery sign. I thought hey maybe it just didn't charge overnight or something and maybe i didn't give it enough time to charge the second time, but no. I charged it last night COMPLETELY and i think the problem is solved. Around 8 am (i woke up at 7), i noticed the battery was already missing a chunk. this NEVER happens that quick. Around 12 it was at 25% of battery. I'm letting it charge now, but is there something wrong with my battery or is it the phone? the phone has been acting up on me aswell (going completely unresponsible, have to hold the home/sleep switch to reset it and just generally lagging up). Should i go to the apple store and try to get a replacement phone?