Working until delivery I need Advice!!!

Ok so I work as a Cashier and im on my feet 8hrs some days a little less. Now that I have hit 30 weeks I am in so much pain with strong cramps on and off everyday, horrible lower back pain that heating pad isn't helping and sharp pain from my butt down my legs making it hard to walk. Is there any advice on how I can get through the days to continue working? I haven't been there long and would like to keep my job for after baby is born.