Possibly spayed female going into heat?

My ten month old female cat seemed to go into heat for the first time recently but I thought she was fixed, her stomach hangs like she's been fixed and when I got her she was with her unfixed brother and never got pregnant. I took her to a cat show this weekend and we were there all day around unfixed males and never went into heat, but as soon as we arrived home she seemed to go into heat. It seems to only happen during the night hours and only for a two or three day period. I have one two year old spayed cat but there are no males in or around my house. Does anyone have any idea as to what could be going on?
Thanks ahead of time.

There might be some tissue they missed and it is making her go in heat.
What age was she altered at?
She will not get preg that way but she might be noisy.

I know some say that spays have a "spay sway" but I have never experienced it and all my spays have tight bellies. I would get her checked out because a desexed cat should not be showing calling behaviour.

I suspect she's not spayed. If her brother wasn't neutered it's not likely she'd be fixed. How old was she when you got her? Some kitties have tummies even when not spayed. Have the vet check her out....if she's spayed they might be able to feel a scar. But chances are much higher that she's just not spayed.

My male has "spay sway" lol.

I bet that she isn't spayed.