Readyfornum2 to me based on what you've said it sounds like that manager was having an extremely bad day, made even worse by being 6 months pregnant, probably feeling rotten, having her assistant manager unwell and being full of hormones.
She may well be feeling horrible about how she was behaving towards you.
I think you may have initially felt like not going back, but that's a fairly drastic reaction to one of those events which seems so awful at the time, but unless it's a regular event, probably isn't something to feel like that about just yet. Hopefully you are feeling a bit better this morning?
I would be inclined to see if you can have a quiet word with that manager. Say you are concerned by how she was behaving towards you, explain that you were doing your best to manage the situation as the assistant manager was unwell and things were difficult. Say you felt you took the appropriate actions during the afternoon and took decisions which were the best you could have done, but felt that she didn't appreciate that and was unnecessarily harsh with you.
Give her an opportunity to apologise, and say that if there is any area of your work in which she feels there is a problem, or you could improve, you would be keen for her to let you know and give you the opportunity to do so as you are keen to progress and improve your performance wherever you can.
That conversation may well diffuse the situation completely, and I would say try that first before you consider anything else. It sounds as though she lost her temper disproportionately in the circumstances and I think she should be given the opportunity to make things right. Be the bigger person and give her that opportunity. She may even call you in for a word to have that conversation herself.
I think also if you are hoping for a promotion, dealing with this yourself is crucial. If you go running to the assistant manager, for example, that isn't the sign of someone who can take on extra responsibilities. As you have found out, managing means taking responsibility, making decisions in difficult circumstances and not being able to ask someone else what to do. So if you can, deal with this situation in a mature manner yourself, it will show that you are capable of dealing with difficult situations calmly.