physically demanding job

I work alone in my department. What i do requires me to be on my feet most of the day. Im 20 weeks and finding it extremely hard to stay awake, and stay on my feet. My back is killing me, im constantly running to pee, and im exhausted. I have zero attention span in meetings and the pregnancy brain fog is hindering my work ability sometimes. Im afraid to ask for them to hire another person to help me because i dont want to appear weak and give them reason to fire me. But i am nowhere near as productive as i use to be. I could pull doubles no problem before, now i can barely make it 4 or 5 hours before feeling like i need to go home. Any other full time working mommas having issues? What do you do to make it better? (This is my second baby and i have a toddler at home, with my first i could go home and sleep but now im taking care of toddler on top of it all and it feels worse this time around).