I concur with the Elfin's corset answer. I wore a modern 'whalebone' corset (you know the real kind that are meant not to go UNDER your clothes) and OMG if you tie it the way you are supposed to you can barely move, let alone breath.
Also, women in the 'olden days' of fainting wore A LOT of layers of clothing. It was heavy and binding and HOT. So on top of the corset already constricting their breathing, they had the extra stress of being overheated and overwhelmed (I'm guessing dehydrated as access to drinking water was more limited and the corseted class hadn't discovered the wonders of detox...practically bathing if you want to get nasty about it)
Further, medical practices in the West were borderline barbaric, especially in regards to childbirth and pre and post natal care. A lot of women were frankly at diminished capacity after their first child and it just went downhill from there. Especially the ladies who had 6 or more kids, I mean c'mon, with the exhaustion and stress of pregnancy and birth and lack of recovery/post care and all the extra effort just to walk and breath and add the extra hassles of cooking and cleaning and living -- yeah, I'd faint too. If only to get a brief respite.
I'm going to going out on a limb and also say that opiate drug use was more common and widespread in fainting days? They were especially given/bought my women as many narcotics were not yet illegal/controlled substances then.
Alright, more legitimate physical reasons to pass out?
Though, don't get me wrong, I like where Aelan and Elfie are taking the discussion as well, I just don't want to be remiss in answering the OP.