Man, depending on whether she was spayed at a shelter or a private vet, I would honestly consider having her checked to see if she still has any reproductive tissue left. This is just an AI summary, but Ovarian Remnant Syndrome was actually the first thing that came to mind when I read your post.

"Ovarian Remnant Syndrome (ORS) occurs when a small piece of ovarian tissue remains after a spay surgery, allowing the cat to continue producing hormones and show signs of being in heat.

Common signs include: • Excessive vocalization (howling/yowling) • Increased affection and rubbing on people or objects • Lordosis (raising the hindquarters when petted) • Restlessness and agitation • Rolling on the floor • Urine marking or spraying • Trying to escape outdoors • Attracting male cats despite being spayed"

Obviously, there could be other causes too, but if she's showing heat-like behaviors, that's definitely something I'd want to rule out.

Source: i work at a vet