What causes unexpected orgasms?

In the past week I have had 2 unexpected orgasms and I don't know why they happened. The first one happened while I was using the restroom ((bad stomachache)) the second one occurred two days later while I was in the kitchen preparing dinner. I know for a fact that it couldn't have been my underwear ((i don't wear any at home...just female boxer shorts or a large T-shirt)) and I wasn't thinking of anything in particular. I will say that they both felt freaking awesome, they just caught me off guard and I have no idea why they happened. I'm open to hearing anyone's theory...