On average, breast-fed babies eat 3-5oz every 3 hours and your milk supply will be about that once it adjusts. You were pumping A LOT of milk per session before and since your baby doesn't consume that much when you are nursing, your breasts are natually lowering the supply to match the needs of the baby. This is normal! And pumping 6oz per session is still alot for the average! Don't be suprised if that supply drops even more to about 2-4oz per pumping session since that is the norm. It is not a bad thing- it is just your breasts adjusting!
The straining during bowel movements is normal as well. She is still learning what it feels like to have bodily functions and what you may think is pain is just her being overly expressive about what is happening. My ped said that as long as the baby's stomach is soft then it is okay. Also your baby can go from pooping several times a day to only pooping once a week! Is that does happen that is another thing that is normal. Eventually your LO will stop the straining noises once she is use to the feeling of pooping.
Remember when you go back to work to pump every 3 hours for about 15 minutes when you are away from baby. I know it is hard.. especially with a 12 hour shift but it is VERY important to keep the supply up! Even if you can do many 5 minute pumps then that could help. As far as the males in your workplace- don't mind them. It is none of their business. Be proud that you are working to give your baby the best food there is!
Honestly, breast feeding is hard for working mothers. Pumping so many times a day while trying to deal with a demanding job and coworkers who don't really understand is aggaravating and WILL annoy you. Don't consider giving up though! You are making your baby strong and healthy with mama's milk!
Also I want to note that if you ever supplement a feeding, that is one less feeding your breasts will not register and that WILL make your supply drop.
You can do this! We are all here for you anytime you have questions, need help, or just want to vent!