Yes, definitely stimming. I'll do things like that... rapping my fingers against my collarbone, for example. Hitting repetitively, slapping or punching oneself... all kinds of things. Sometimes frustration, sometimes because I've got a lot of energy with nowhere to go.
If you have a stim like this, remember that autistic people's nervous systems sometimes do not register pain correctly, which means that you may do damage to yourself without realizing it. Make sure to check yourself for any injuries. Superficial bruises are okay if you don't mind having bruises; scratches are okay if you keep them clean and let them heal. I often find bruises I can't remember getting when I take a shower (and, no, I don't have a bleeding disorder; I'm just clumsy and don't really notice getting hurt).
By the way, NTs often freak out about stims that cause injury or even things that look like self-injury but aren't--like punching yourself without leaving a mark, or chewing on your skin without causing bruises or breaking the skin. If they see you, they might worry, or they might put you on medication. Some NTs understand, but most of them haven't got a clue, and some of them actually get scared that you're going to really hurt yourself. They're usually wrong, but they don't really understand, and they don't know that they don't understand. So you might have to deal with some of that.