I started running last week. I am doing a couch to 5K program from Beginnertriathlete.com because I want to be able to run my sprint tri in November. Today was the 3rd week. Last week I ran 3 minutes and walked 27 minutes and everything seemed fine. This week I am to run 6 minutes and walk the rest. I did my run/walk this morning but now my lower back has pain.

Is it becuase I need to build up my core? or does my body just need to get used to the running?