It is all about being emotionally vulnerable. If the girl is dismayed to find herself there, or doesn't want to be there with you, you'll find out about it. Otherwise, you should realize that she is already trusting you to 'protect' her, not by preventing her from feeling vulnerable, but by allowing her to to feel vulnerable. And you shouldn't worry about intruding. Remember it's about her feeling safe With You, enough to let this out from inside.
And of course all of those goes the same for men, I have cried before as well. The first time I visited my girlfriend (rather than the other way around, long-distance relationship) we were alone in her basement and we'd already had some fun, we were talking and I started crying helplessly, it was extremely old and deep, I was sobbing for ten or fifteen minutes and my ribs hurt the next day. She couldn't 'protect' me from letting out the old grief though she tried to a little bit, we were holding hands and I told her "I know you're trying to help, you do help me" and saying that I loved her. I was so relieved to have 'found' her, because I thought (from an early age) that I might never find anyone to be with, and now that I had, I guess I could let that all go. At the time she was frightened, she had never seen me so overcome with grief!
To get technical about it, I was releasing tension held in the intercostal muscles (think of barbecued ribs, only human) and the connective tissue around the ribs and lungs.
If you'll allow me to geek out a little here, I'll tell y'all a little about fascia. (da da daah!) You see, the connective tissue that runs like spiderwebs or saran wrap between all other tissues in the body grows around our habitual positions. Which means if you are holding your muscles in a certain way or posture, new fascia that grows will attempt to support it that way. Like bone or muscle, it grows depending upon the stress placed upon it by the rest of the body. The good news for chronic posture/movement problems is that you can move fascia around fairly easily with manipulation (Rolfing, Hellerwork, etc.) even if it's not moving on its own.
When you 'hold in' emotions you really are trapping them somewhere in the body, with muscle tension. Think of stopping yourself from crying, yelling or laughing inappropriately... what muscles are you holding still, during this fight against your natural expression? Eventually, the fascia grows around this tension, so that even if the muscle cells can lose their contraction, they are held in the habitual 'tense' pose in minute ways. It builds over time.
I have had numerous experiences of these emotions bubbling up out of the body when fascia is moved or begins to move on its own! This stuff sometimes will just come undone, like peeling apart layers of saran wrap. For example the periosteum (a thin shiny layer of connective tissue) around my right collarbone unwound itself once during a massage, leading me to laugh helplessly, it was laughter that I was not 'allowed' to express at points in the past, and the deeper I laughed inside, the more my clavicle moved. When these emotions leave it is a release of physical tension. (Physical in the sense of bodily, and also in the sense of, uh, physics.) What, you didn't think those pains or heartbreak around your chest were 'just nerves,' did you?
People are good at ignoring chronic emotional pains like this because we can hold them in place in the short term with relative ease.
This is one reason why we feel more 'free to move' after a good, vulnerable cry/laugh/rage, because some capacity for subtle movement has been freed up.
Anyway......... all jokes aside crying after sex just means vulnerability, it doesn't mean you did something wrong. (I'm assuming here that you're not an emotionally blind sex-user of a person
) Just listen to her if she talks about it, and show some vulnerability yourself, or strength as the case may be. I firmly believe that vulnerability equals strength.
