If I sound like I'm not taking this seriously then I greatly apologise. I'm terrible at tones and I assume my writing/speaking style comes off as disrespectful but I swear I don't mean it, I'm just shit at putting my thoughts into words for others to understand, especially anything to do with sex.
Never happened to me at all but I assume it's like the pain of fucking up while trying to fuck yourself with something you really shouldn't be shoving up there/shoving into. Those parts are really really sensitive so every little thing can be felt and combine that with physical force, emotional shit going all over the place and yeah, safe to say it does not sound pleasant at all.
It's hard to make an educated guess as there's many situations where sexual assault of this kind can take place, but I assume you mean what people first think of: a woman being forced physically by a man and she's fully conscious.
First there's the emotional part. You would be feeling very distressed due to the situation, terrified that you were going to essentially be tortured (and maybe a fear of side effects such as STDs), violated for obvious reasons, angry due to said violation and potentially even more depending on the person and if they had any prior problems/experiences relating to the topic.
Next is the physical pain. The vagina naturally makes this lubrication stuff when it's horny (I think it's also called cum but nobody really talks about this kind of stuff so idk) but if you're not aroused then there's no liquid to make it buttery smooth up there so not only would you feel it much more, but it'll sort of be like trying to shove your fingers in your throat when your throat is completely devoid of saliva. Again, this is just an educated guess, I may be completely wrong here, but the inside of the mouth kinds feels like the inside of a vagina just less sensitive. Coming from someone who uhhhhh...knows how uncomfortable and painful that can be from my own stupid actions when I was younger (it also opens up when aroused aswell, forgot to mention that, so it'll be all tight), I imagine it'll be like that but worse because you have no control. In this hypothetical, the woman is being forced, probably being held down by a man who's much stronger and heavier so there's all that weight and the distribution would be mostly on the wrists I imagine, or the ankles. A lot of stress on quite bony parts of the body is gonna hurt, especially if you're trying to break free. Also if they're being rough then there's also pain from actual organ damage which is a whole other thing that I don't want to spend too long thinking about because I'm too much of a pussy to detail what I think the feeling of a vagina with cuts and bruises would feel (probably hell and like gums but even more stinging). I don't know about how it would feel if the roles were reversed as I don't have a cock but I suspect it's a similar story as the bits aren't that different biologically.
Another factor that could be included is if the body decides to fuck you over during this so not only would you be extremely uncomfortable due to well...that, you also now have to deal with that disconnect where mother nature's going "yes" but you're brain is going "no no no no". I don't know if that happens. I assume it happens because sometimes mother nature just does that. Either way, does not sound fun at all.
However, there's one last thing I think I should mention. The kind where you're drunk as hell and you wake up with no memory in someone's bed. I think you can imagine the sheer horror.
Anyway this is my educated guess as I doubt many people are gonna come forward and tell their story as it either unlocks a traumatic flashback, they werer drugged or the brain did some self preservation thing and tried to forget about it as much as possible. I assume if you want to get an idea on what it would feel like (I don't know why you would want to physically), just really fuck up trying to masturbate when you're not horny at all and times that feeling by 7.