I accidentally went deeper into my girlfriend, and she liked it, but I don't know what I did.

My girlfriend and I were in missionary. I was going at the usual pace for us, not particularly fast or slow. Then my girlfriend asked me how I did "that" and I didn't know.

She says that for a few pumps she felt like something readjusted down there every time I went all the way in. Not her exact words but that was the sentiment.

Basically, she felt like I went deeper than usual by going through some sort of tight spot down there and then the head would "pop through" and she enjoyed how it felt.

I honestly have no idea what she felt because I didn't feel any readjustments and I wasn't doing any specific technique or trying for a specific angle. I was kinda just pumping as usual.

Any suggestions as to what may have happened and how to replicate it?