A similar thing happened to me...when I was a teenager and in my early 20's, I got into a habit of cutting my gums until they bled. I don't remember deciding to stop or actually stopping, but I did stop and didn't give it another thought until about a year ago when I was having a tough time, and some difficult feelings had come up, and then before I even knew I was doing it, I'd done it. It freaked me out, partly because it was an old behaviour seemingly coming out of nowhere and partly because I didn't consciously do it so it felt out of control. I felt pretty horrified that it had happened. I didn't tell my therapist.
About 6 months ago, I went to the dentist for a hygienist appointment. I hadn't been for a few years so it was a bit hardcore - lots of bleeding. And it was like it flicked a switch in my head and I couldn't stop thinking about it and couldn't resist the urge to do it. I was doing it every day, several times a day.
I saw my therapist a few days after I'd been to the dentist and she randomly started talking about flossing and I said I didn't and she looked shocked and asked why not. And I just said that I had this bad habit when I was younger, so, slicing my gums up with floss didn't feel like a comfortable thing to do. And she was asking questions about it and was looking concerned and then I just sort of blurted that I'd been doing it again. So, she started talking about "self harm" and I got all defensive and said that's not what it was, that she was blowing it out of proportion and that, no, I wasn't going to phone her every time I was about to do it...
It was a horrible conversation to have and I experienced deep, deep shame when I told her. And then I had the worst experience of dissociation I've ever had!
Now - I still do it but not every day. And we don't really talk about it, though it is "out there" that I do it. Occasionally, if I've done it more/worse or if it's more on my mind, I say something. And I'll do it by saying something like "I can't leave my mouth alone at the moment".
Like you, I don't think it's "a big deal" in the grand scheme of self harm. I'm not badly injuring myself, I can't accidentally kill myself by doing it, there are much more serious, risky things I could do to myself instead but I don't... And yet, I am still doing things to cause pain/draw blood and I appreciate that isn't a very healthy coping mechanism! So, while I hated having the conversation with my therapist, I do think it is A Good Thing that she knows where I'm at with it.
Can you say something like "I found myself doing X the other day, which has unnerved me because it feels like I've crossed a boundary. I think it's only minor and don't think either of us need to be worried about it and I really don't need a talk about keeping myself safe - I just wanted to put it out there as I don't feel great about the fact that I felt the need to do that."
Or something?!
Good luck with it :-)