I have that as well, but I usually have a full bladder when it happens. It usually only happens once or twice a month.
Hi, you don´t have to worry, orgasms in sleep happen also to women, but it is not so well-known or so discussed, but I have read some articles and sexology publications where it was mentioned as a normal thing. One reason why it has not been so discussed can be connected with the fact that in case of women there is not so "tangible" evidence of orgasm as in case of men (ejacultaion, wet bedsheets etc). I have also read a case of a girl who inserted dildo into her vagina before going to sleep in order to induce such orgasms and to make them more intense.
Don´t worry, your experience is absolutely normal, probably it can be viewed as a sign that you are healthy in terms of sexuality.