21 weeks pregnant and seriously stressed out with work?

Ok so I'm 21 weeks pregnant and finding it really hard to cope working full time. I'm a retail concession supervisor/ manager and I'm souly responsible for my concession. I have only 3 other part time staff members to help me out and I'm finding it extremely stressful trying to meet daily targets and doing all my daily duties as I find myself on my own a lot of the time! I stand 4 8 hours a day and have never had a day off sick so I'm very hesitant about asking a doctor to sign me off work but I'm feeling it may be the only option to ensure my baby is ok! I'm scared of leaving my boss in the lurch (who is based in another store and rarely see) with not enough cover to run the concession. I've also found that I'm losing hearing in both my ears and its very difficult for me to hear my customers and help them out so that has worried me a lot aswell.. I'm just in a constant panic and can't enjoy my 2days off a week because I'm constantly getting phone calls from work about problems! Ahh I don't know what to do, this is my first pregnancy and need some advice to help me through. Thanks ..