My mobile phone suddenly started to drain battery at an extremely fast rate (ca. 1h duration, instead of 8-10 days). Any hint on what could be wrong and how one could fix it? Thanks in advance!!!

1) The phone is > 6 years old 2) I got a new battery today, but same result, which means the problem comes from the phone. 3) The problem started right after I have been shooting a few pictures with flash on, though nothing I had not done before or unusual. 4) At the end of the charging phase, I get a message saying approximately " Serious failure - Stop using the phone and contact Sony Ericsson Customer centre". 5) The battery becomes really warm both during charging and while in use. This applies to both the olb battery and the new one I just installed.