If stalking is illegal, how is it possible for anyone to hire a private investigator to track you just because someone wants to?

Background: My husband and I cut contact with his mother in 2017 due to a very violent response to establishing boundaries. Very quickly after that her husband, her daughter, and her were slandering me (mostly) to everyone they knew. She lied about what happened and things I said (I was actually silent almost the whole time and when trying to respond she went off and told me to get the beep out of her house). The situation was awful. A friend of hers contacted me to tell me that I was taking him away from his mom and her, "his second mom", which was never the intention, but just to tell her we can run our own life and want her to back off some. MIL texted me telling me she's "onto me" after we left and madr a veiled threat.

She said she hired a private investigator to find us. She gave us multiple different details that contradict how she really found out we moved, but said one of the ways was that our utility bill had gone into his name in another state we moved to and kept fairly quiet about moving to. However, there were no utility bills in his name, except our phone, but that wasn't changed over in our new address until April, two months after she came to the new state and waited for my husband outside at a gas station that she "naturally knew he'd stop at" to not "disrupt his workday.

We are suspicious she may have used his social security number to find him honestly because from my understanding she used his social for a car loan and other bills when he was little.

Is this stalking? Again, we've gotten different answers on how she found us, but it doesn't seem right that people can't live their lives without being watched just because someone wants to track us