I was going to ask about possible bullying, but then saw your update - my own DD who is also 10 went through this the last couple of terms, so I feel for you as it's heartbreaking to see them so sad - initially she didn't own up to what was going on, as it was actually friends who were bullying her - weird loyalty thing going on -
like you I wondered about hormones, but turned out to be a combination of lack of sleep - she was worrying & not sleeping well because of the bullying & felt very isolated because of the bullying - could there be more going on than you realise?? - is it possible that like mine she doesn't want to worry you - especially as you are pregnant?? - it might be worth keeping an eye on - mine crumbled & it all came out after I pushed her to speak up when she was upset & angry at me for no reason - she broke down, sobbed hysterically & it all came out.
Does your DD have any out of school activities with none school friends? Getting My DD socialising more away from school, encouraging other friendships helped my DD a lot - thankfully after an initial blip, her school were also good, but her head did tell me that this is the age all this stuff kicks off & boy has it kicked off amongst my DDs crowd - at different times each of her once long time good friends have tried on the bully side kick hat & DD has been the target because she has refused to bitch about others - one girl has always been the ringleader, but the others have all been manipulated & have been involved too & apparently these days, this is quite normal at this age - so definitely worth keeping in mind
Thankfully after a few hellish terms, my DD has come through it & it now rolls off her back far better, also most of the others like her seem to have really matured & have come out the other side it - they too have been targeted by the ringleader at different times, & what was once a bigger group of friends is now 2 groups, one small very bitchy one & one larger one of good natured girls who generally rise above & ignore the bitching & are generally getting on really well & it's a lot to see them so happy at school & not getting upset by the bitching
I'm rambling as its late, but in short, this is the age when their friendships can get very fraught - borne out by teaching friends & family & DDs head & teachers, so I wouldn't be at all surprised if this is still the route cause
Good luck, I hope you get to the bottom of it