You should google a site called Healthy Strokes - it explains in detail why prone masturbation isn't a good way to set yourself up for enjoyable sex. A lot of the material is for men, but there's info for women too.
I used to masturbate prone, but I changed it up in my late teens. I still tense up my ab and pelvic muscles during sex - it may be that yours are a little weak from being accustomed to a less-strenuous position. I like to have my legs out straight for clitoral orgasms, but I'm more flexible with vaginal orgasms.