What is Actually Happening Inside my Abdomen when I think of my boyfriend and feel butterflies in there?

When I think of making love to my boyfriend, I get this feeling for a few seconds in my abdomen, like a wave of intense feeling, almost like butterflies. What is actually going on down there? Is it uterine contractions in anticipation of sex or something like that? This only happens to me when I'm deeply in love like I have been for the past 3 years with my current boyfriend. When I have fallen out of love with someone, like I did with previous ex's, I no longer get this feeling. This can happen to me any time I think of intimate moments my boyfriend and I have had together, whether he is anywhere near me or not. I can be doing perfectly normal things, like working, shopping or doing household chores, and a thought will run through my mind and I wilI get that wave of butterflies in my abdomen for a few seconds. I know it's caused by arousal, but I'm just wondering what is actually physically happening down there to cause the waves of physical feeling. I can't find anything on Google. Thank you for any responses. :)