Hi R., I'm thinking this is a one time thing, because she seemed particularly protective so I'd blow it off.
I'm going to play the devil's advocate here. When I'm at the park, I'm very active with my daughter and I climb and play with her on the equipment. One of the most annoying things is when other children bide for my attention because their parents aren't interacting with them. I'm not saying that you are that parent not giving your kid attention, but kids see a parent interacting and want a playmate to "Watch me!"
Perhaps your son interacted with her first and she was playing a game with him. (My daughter loves when I pretend to be overly protective and she loves the praise I give her when I realize that she can do something that I was pretending to freak out about.)
The other option is that perhaps she was trying to get YOU to give your son attention somehow so she was being overly annoying to get your attention so show you that SHE was giving your kid attention instead of you. Almost like she was picking a fight with you.
Also, I personally have had to step in when other children step out of line at the park (like bullying), while the parent is either no where in sight, or just ignoring their children. I stepped in once when I saw a young toddler walk in front of a moving swing, which they would have gotten hit had I not stepped in. Where were the parents? The park was that busy that I never saw that little tots parents. I personally think that some parents aren't protective enough. Just something to chew on. My bet is that she was thinking you weren't interacting enough so she was trying to get your attention to get your son away from her, by you giving him attention instead of her.
Just my opinion, no offense intended.