Why I sart getting "Fatal error: Allowed memory size..." after php upgrade?

The hosting company I have one account (and several Add Domains) upgraded php on the server from 4.4.7 to 5.2.5. After the upgrade I start getting the error message: Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 122880 bytes) in... on domains they use Gallery2 or are CMS (Joomla or Drupal) based. I contacted Tech. Support (Live Chat) and he told me I have to edit memory limit in php.ini (for each domain). I asked him why it started so suddenly, did upgrade caused the problem and I got as an answer: "Your scripts have begun to use more memory. That would be the only reason for this error." It doesn't make a sense to me and sounds like "let's blame something else" type of answer. I'm going to edit php.ini though what if "client" doesn't have a clue what's php.ini or how to do it? I would like to hear your opinion what caused the problem. I'll edit php.ini, I'm not going to make a big deal of it, though, I just want to know why it happened. Thanks for help. -afan