I don't know about mood, because my mare was pregnant since I bought her. She is very, very sweet, so I hope she stays that way after she foals, lol!
I am guessing that being pregnant stabilizes their mood because they aren't going through heat cycles.
So I don't know if you can normally tell if your mare is in "season" but that is something you can watch for. Apparently some mares really act moody and show it, but others you can't hardly tell.
Wish I could be more help! I am just learning my way through this process too! But it sounds like she was kind of early to tell definitely whether she was in foal or not, so if you wait a while and try again, you should know for sure.