Mama cat is acting really really weird

For the last two days she has been desperately trying to get out of the house, up to and including attempting to open my window, because of this tomcat calling outside. He's been circling the house calling and she's been following him around on the other side of the wall, calling back. All night -_- Making this 'brrrrr-ow!' noise, mostly. It seems to me she's acting like she wants to get outside and 'be with' him, rather than fight him away. My first instinct would be that she wants to mate, but she's been spayed. Does she just have a weird way of showing aggression? 

And now she's hanging around me, making this short chirrup/purr every few seconds and rubbing her face all over everything. Unless you go near a door, and then she begs to go outside. 

It's exhausting to be around, and it's bizarre. She hasn't done this before (except sometimes the calling at night, which hasn't happened for a good couple of months) and it came on very suddenly, like flipping a switch. She doesn't seem to be in any pain or distress except for the frustration of being shut inside. Can anyone shed some light? 

I would take her in to be checked if she has been spayed a while ago as I would worry about a hormone issue if she was calm for so long.  

Yes, she was spayed about 6 months ago. She was pregnant when I adopted her and I had a spay/abort so she didn't have any (more) kittens, and she came back looking like she had been deflated. So I know she was spayed. What kind of hormonal issue could we be looking at? 

There always is a chance some tissue was left behind but with that I would have thought she would have showed signs sooner.  Was she is very poor shape when you got her?  Was she normal weight?  If something is causing her body to cue her into "being in heat" such as a cyst or such.  Was it a shelter vet or your own vet you went to-I would try to have her seen by the past vet in case they want to open her back up and double check it normally would be done at reduced rate or free.  If this is the case it is not the vet was bad just that a pregnant spay is harder and these things do happen.  

She was in very poor shape when I adopted her, she and her 2 babies were strays who'd been terribly neglected and abandoned. She got pregnant young and had almost no food, so she's forever the size of an 8 month old kitten, and I had her spayed within a week of adopting her when I realised she was pregnant. When I had her spayed she dropped to below 3 kg (6.6 lbs) partially because she's just so small and mostly because she was still recovering from starvation. Now she's much healthier and has been for a while. Is it worth mentioning that where I live it's the middle of winter?? 

I've never seen a cat in heat but I wouldn't be surprised if this was it. She's making these mournful deep meows too, and I can hear the tomcat making the same noises back at the front door. I had her spayed with the vet just down the road with me, so I can definitely go back to them. I'll take her in on Monday and talk to them about it. Unfortunately I can't get to one over the weekend. 

It could be that since she's only been spayed for 6 months she's remembering that it's mating season (cats in the US usually start mating season in January. . .and I think July is the equivalent in the southern hemisphere). But do have the vet check her out. Being that she was pregnant when spayed, that might increase the chances of them missing something.

With it being winter that VERY well could be  why as she recovered she never went in sooner and now that she is healthy she is in heat in time for your regular mating season.  Keep your guard to not let her out.  Disease can be passed from mating and sometimes injuries happen.  I would think they will open her back up and explore if something is still there.  Does not take much left behind to trigger this.

Keep eye on her food intake as cats in heat tend to not eat as much so want to make sure she does not lose any weight.  Also her personality may change until she is out of heat and she may be friendly but then very agitated so keep that in mind.

I will do my best to make sure she stays inside (and that my flatmates don't let her out) while this is going on. It's hard to keep track of how much she's eating because we also have a kitten from her first litter, and they eat the same food and from the same plate. But I will notice if she loses any weight because she's already quite naturally slim. 

How common/rare is this incomplete spay? Will the vet think I am crazy or some kind of feline hypochondriac if I contact her with this? 

Mama cat actually seems to have calmed down now, I've just got home from being out all day but she's not yowling at the door and she's acting like herself again. Of course there's no guarantee she won't start up again tonight but I'm enjoying the peace for now! Hopefully this is the end of it, although I will probably still need to take her to the vet. 

 I never had it happen or know anyone in real life but have read about it here and there. It is not much needed left behind to trigger it.  A pregnant spay is harder as blood vesels are bigger.  It is also harder recovery for the cat as scar will naturally be bigger.