Advice?

Hi I'm 21, and 19wks 5days pregnant with my first child, I am a cashier and work 8 hours a day 5 days a week. 
Most of the day (other than two 15min breaks) is standing behind a till. I've read online standing in 1 place can be bad for the baby as well as getting cramps (my legs, back and ankles ache sometimes but not sure if that's linked). Only way to be allowed a chair is with a doctors note, do you think it's okay to keep working as it is or should I see my doctor :/