Scarleteen

I am a (vagina-having) virgin. I've used tampons for ten years, I've explored with fingers before, and I have had two pelvic exams, so penetration is not exactly foreign to me, but yesterday I tried a dildo for the first time. I count the experience as an overall success, but I did encounter a rather odd sensation that I had not expected, and I am curious to know if there's an anatomical explanation.
The first four inches or so went in exactly like everything else I've ever inserted, just more slowly and with slightly more difficulty due to the larger diameter. However, at that depth I seemed to hit a stopping point. It didn't hurt and I know my vagina is longer than four inches, so I kept trying and eventually made it past that point, which was when the odd sensation occurred. It felt like the head of the dildo had made it past an area of smaller diameter, and once it did, the whole rest of the length went in without any resistance. The only thing I can think to compare it to is sliding your foot into a boot, where your heel can get caught at the ankle but once you get it past there's no problem. After the initial insertion, removing and re-inserting the dildo was not difficult or uncomfortable, but each time I still experienced that sensation of passing through a tight spot.
I've never heard of anything like this, so I'm very curious to know what caused it. (To be clear, though: I'm only curious, I am not upset or worried and I am not seeking medical advice, just general anatomical information.) Google isn't helpful; it only gave me page after page of myths and myth-busting about vaginal tightness. Scarleteen was a helpful resource for me in college when I was struggling to come to terms with my sexuality, so although I'm now older than your target audience I decided to bring my question here.
Thank you in advance for considering my question, and whether you have an answer for me or not I'd like to thank you for the work that you do.

Thanks so much! Glad we've been helpful for you.

You know, the boot analogy is pretty right-on for what I think happened here.

So, not to the degree of angle you find ion a boot, but similarly, the vagina isn't a straight canal. Rather, it's actually curved, a bit like a banana. So, if we (or a partner) are pressing anything straight into the vagina, often soon enough, we're going to basically hit a back wall rather than continuing upwards into the canal.

Too, once you are inside the vagina by four inches, you're pretty likely to be working around your cervix a little, so there will be less room. When arousal happens, the cervix pulls back some, which is why you even have something like four inches of space. With even more arousal, it can pull back even further, but on average, you're still only usually going to be dealing with about six inches of length there, cervix included.

Make sense?