My suggestions: Look at the bottom of your sandals -- is the sole worn unevenly? I wear Birkenstock's all the time and as soon as the heel portion starts to get worn down my feet IMMEDIATELY start to hurt -- also my legs, ankles, knees, hips, etc. (I'm having my first fusion next month so hopefully I can say "Bye Bye! to the pain!) I MUST have good support because I tend to over pronate -- for me Birks work well.
Also, what quality were the sandals to begin with? I can NEVER buy cheap shoes -- my feet start to hurt right away. Make sure you have good supportive shoes -- ESPECIALLY is you do a lot of walking in them. (I use my Birks for walking shoes -- my athletic shoes irritate the bone spurs I have on the tops of my feet: OUCH!)
Hope this helps...