Working while pregnant?

i'm 27 weeks pregnant and i work at sonic. for anyone that doesnt know what sonic is, its a drive in food spot. i am constantly walking, on my feet all day, carrying sometime heavy trays. My doctor said walking is great and she suggests it but im worried so much will send me into preterm labor? i feel my son really low already & i get vaginal/rectum cramps sometimes..Just wanna know if Anyone has had a hard labor job and still went to term perfectly fine. or any opinions. Thanks!