Identity change = alter. That's the definition of an alter. An alter is an alternating identity. Other people call it a "kintype" or "theriotype" when the dissociation level is low and the identity is non-human.
They do not always feel like "somebody is inside my head". It really depends on the intensity of the dissociation. They can feel like "I am turning onto somebody else temporarily" when the dissociation is less intense.
Just to add a bit of nuance: this is not necessarily an alter, but that kind of thing is 100% a dissociative part. Alters are "just" the end of the spectrum when you look at dissociative parts. They are the most dissociated ones.
You can have a lot of dissociation, and dissociative parts with clear roles, but they don't have a clear, strong sense of self, and not always trigger blackout. Depending on the exact symptoms, it can be OSDD-1 (DSM-V) / partial DID (ICD-11), or DID.
I'm officially diagnosed with partial DID (even if my T agree I had a DID presentation until my early 20, but it's too young for it to count for the diagnosis tool), I have clearly defined alters with a strong sense of self, but no amnesia on a daily basis (other than the memory issues caused by ADHD). Right now, my amnesia is occasional, linked to the moments when my window of tolerance is overwhelmed (in DID by the ICD-11, amnesia happens on a daily basis, and is primarily linked to the identity changes/switches).
I totally relate to the "it's like becoming someone else" when another alter front. For me, that's how a switch started to feel 10 years ago, when the dissociative barriers started lowering because I was officially out of the reach of my abuser ; and it became our norm 7 years ago, when we totally stopped having blackout. This is how a switch with enough communication and without amnesia feels for us.
When this happens, one alter can be "stronger" than the others and their emotion "overcome" the whole body and mind. You can feel like you're becoming that alter, but if you pay attention, your emotions and own reactions are still there. Sometimes, two alters are equally affecting the body, and their emotions are all felt and expressed by the body. It can be very confusing when it happens, specially if it happens while you're triggered.
I know that people can have these heavy kinks without any kind of trauma, but in the context of my life and what I know now, I can't help but feel like it definitely relates to my trauma.
I would say: trust your instinct, protect yourself, take your time to sort this out before doing anything.
I had kind of the same issue, but I didn't take the time to sort this out, I went with denial and the idea that everything is fine for months/years, and now I regret it.
When I'm physically intimate with someone, I don't feel anything between the waist and the knees - and I'm hypersensitive to touch, so this is very much not normal. My first experience was with someone very respectful and understanding, and it went well. Actually, it's our only positive memory with being physically intimate with someone else, the only time no one was triggered.
Then I met my ex. 5 years ago, I was still amnesiac and in denial of my sexual traumas, and since everything went okay the first time with the other person, I never questioned my lack of sensibility. My ex didn't question anything too, but make it a personal goal to find a way to make me feel something. We never had to explore anything outside vanilla sex to find something that was making me receptive, but I discovered years later that it was 1) what my abuser did to me, and 2) retraumatizing my young alters.
Finally, even if I was enjoying what happened on the moment, I started to dissociate more and more during intimate time, to have amnesia about those times (but it was super-precise, so I didn't notice it at first), and ultimately, even "this thing" she found stopped working. After that, I started to have freeze responses every time my ex tried to initiate intimate moments. She wasn't smart about it, and the level of frustration she expressed was the icing on the trauma cake.
Now, in your case, I can't say anything since I'm not in your head. But I know it's hard to face the probability those things are real, and even more to take the time to find out what is really going on. You have all my support in this journey.
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French person with ADHD
Former partial DID
Functional multiplicty, highly integrated