My phone dies every time I try to turn it on?

I bought this phone in June, and it worked great up until now. One day it had full charge and it just died on me. I thought it was a temperature issue, so i waited until it cooled down. i turned it on, and it showed that i had -17% charge (which was so strange), even though it had full charge earlier. So, i charged it and turned it on. It worked for a few minutes and the battery just died on me again. Now i can't even turn it on without it dying on me. When i try to charge it now, it just reboots and dies over and over again. I went to the shop with it, but the guy assumed it was a software issue ( even though he didn't even do anything with the phone). I am certain this is a battery issue, but i don't know how to get it fixed. It's a shame because the phone was wonderful before this, and i don't want to have to get a new phone.Any suggestions would be great.