Assault or not?

I'm not sure whether this would be actual assault, and whether I'm in denial about it.
But about a year ago I was in a short BDSM relationship, even though it was more of a fling.
After meeting up with him once or twice, I told him that i didn't like him using needles on me (like piercing my skin with them) but once when we met he tied me up and still used one on me. I pretty much crashed, struggled and told him to stop, but he still kept going.
When he was done he untied me and seemed shocked by how badly I was dealing with it. Then I pretty much cried in his arms till I felt better.
I know this isn't directly sexual, but I'd still like some opinions on whether this would be assault or something and wrong and whether I should've done something about it, or whether it was my fault for getting into that type of relationship in the first place and not being able to take the pain even though I felt I should be able to (but then again, I still told him not to and he didn't listen).
The thing is, I'm pretty sure he thought I enjoyed pain (which I actually don't), and that he was "supposed" to push my boundaries, or whatever, and that no doesn't always mean no, or stupid stuff like that. Plus, I once read that in BDSM it's considered nearly "normal" for the sub to crash and cry afterwards. That's the thing. Should I have done something? Am I deluding myself in thinking that he didn't do anything wrong because in that kind of relationship, it's pretty much expected and the dominant person can do whatever they want anyways? I mean, in theory, it's always wrong if it's not consensual, and that definitely wasn't. But ummm... I don't know, it's hard to grasp that maybe I should've done something about it and shouldn't have reacted the way I did. Even though I think why I reacted that way, but still... is there any way to actually excuse what he did? Or am I wrong in defending his actions?

Yes, that reads as actual assault to me. I'm sorry it happened. You did what you needed to do in advance, which was to set out your boundaries and to be clear about them. And when he pushed past them at the time, you told him to stop. That's all you could do, and it should have been enough.

There's nothing wrong with BDSM or relationships that involve it. As in all relationships, however long or short they are, communication is key. You held up your end of that, he abused you anyway. Yes, it's true that "no doesn't have to mean no" -- if that is negotiated in advance. But in that case, there should be another safe word to use. If one isn't prearranged, then No should be enough. And safe words are SO important because I don't think crashing is ever a positive part of BDSM play. It suggests to me that a scene went too far. Sometimes it's hard for subs or doms to know their boundaries without going past them. So I wouldn't say that a crash or boundary-push is automatically assault. But in this case, he had the relevant information and ignored it. That's a problem.

It's not true that the dominant person can do whatever they like. Really, the sub sets the boundaries. The dom may explore and push at those, but except in fairly extreme [pre-negotiated!] circumstances, they're still always limited. Because it is meant to be fun for both / all participants. And hurting someone without their consent and enjoyment, is simple abuse.

It's up to you how you think about what happened. It's totally up to you. I don't think there's a "correct" way to view it from a moral / ethical point of view. I think that excusing him to any extent still doesn't let him off the hook for having done the wrong thing. Nor would it mean you agreed to it somehow. And definitely wouldn't mean you wouldn't want it to happen differently if it came up again. But the way you've described it, it doesn't seem like he was evil or manipulative. Just terribly stupid. So I can see how it might not help to directly label it as assault.

Legally it was assault. If he penetrated your body in any of the commonly accepted sexual areas (breasts, genitals, buttocks, etc.) it was object rape. Regardless of anything else, in matters of law, the moment someone says "stop" or indicates that, anything that continues beyond is rape.