Some context: I have two cats, one male (Gatão, 3-5 years old) and one female (Pitica, 1-2 years old). Pitica has been spayed before I took her in, Gatão has been spayed since last week. They came from the same house, so even though they tolerate each other, I wouldn't say they are friendly to each other either.
The issue I'm having started two days ago. Gatão has been acting weird towards Pitica and meowing a lot. I observed them yesterday and today and I came to the conclusion that he is "keeping an eye" on her. For example, he is always following her around. When she sits or lies down, he also sits or lies down in a position from which he can see her, and he watches her from there. When she is sleeping inside the house, he sleeps too, but when he hears her go out, he wakes up and follows her. When she leaves his field of view, he looks for her desperately and meows continuously while doing so; when he finds her, he stops meowing, sits and watches her.
He does this all day long and she seems extremely stressed by it. She hisses at him and runs away from him, but he never stops following her around.
What could be happening? Should I be concerned? Will this behaviour fade away or should I do something to help them?