Battery running out too quick

I own a Ti-84 Plus CE Python Calculator, which I bought it about 2 months back. Calculator is really great, held battery for about 3-4 weeks. Recently, we had a math project in class, due to which, I didn't use my calculator for over a week. It had a full battery before the project started, and had 0 battery, the week later. I charged it, and took it to school and after that week, the calculator had 0 battery. What could be the cause of this? Do I have to change batteries?? I heard from other posts that Ti gives a free battery replacement. Any tips would help.