Contractions in the absence of other symptoms are not an early sign of pregnancy in anyone. (I assume that if your stomach had been growing noticeably and didn't stop, or you were having morning sickness or sore breasts or feeling kicks, you would have mentioned it here.) In pregnancy, all those things and lots more happen months and months before contractions.
Does this (the contractions) also happen when you have an orgasm? That's actually pretty common. It's also not uncommon to have an orgasm when doing certain kinds of exercise (such as climbing a rope). It has to do with the pulling and straining on certain muscles. Can you tell if these were contractions in the uterus or the vagina? Both can contract during orgasm.
It's also possible something hormonal triggered the contractions you felt. You could go see your doc and ask for a hormone test and also an ultrasound, to check for fibroids or anything else the doctor suggests.
Are you positive it wasn't a muscle spasm? When not pregnant, they call these contractions just cramps. I've read that uterine fibroids can cause this. When was your last pelvic exam?
It could be some sort of abdominal/gastro thing. I have IBS and the pain associated with cramping is by far worse than my period cramps and makes me sweat bullets.