Hey Starfish,
I agree with other replies, it is a lot to take on, being a new mum, study and then work. I am amazed at how you are coping so far! I think you need to give yourself a break. We all stuff up at the best of times, and to get a few things wrong, after returning from having a bub, expressing, dealing with an over-zealous and competent temp, etc, just adds to the pressure of work.

I agree, chat with your boss, tell her you are aware of your feelings (hormonally) and recognise that whilst they do have an impact, they are not the reason for all the things that are going wrong. As you really want to keep the job, it is great that you are making an effort.

Most of all, be kind to yourself. Ask for help, and like the others say, just walk out if you need to express. It is THEIR problem if they don't like it, not yours.

*hugs*