As far as pain ive never seen a bitch become physically uncomfortable.
Mabel did get hormonal, for her she was sweeter to people and wanted other dogs to disappear..
This isn't the case with all bitches though, some don't change, some get moody in good or bad ways, its hard to say. One bitch I know is usually just "upset" and while still tolerant of people and dogs she's just not herself and seems depressed the first few weeks.
I've also seen some bitches break with diarrhea or upset stomachs at first.
It wouldnt hurt to put her britches on now too btw and do some positive shaping so she's used to them ahead of time.