I went to school with a girl who had BO that smelt like onions. It turned out that it was, in her case, because she didn't wash the old sweat and deodorants off properly before applying the next lot and it kind of built up and smelt like onions. I am not suggesting that you do not wash/shower properly though and it could well be something to do with your hormones. I sweated a lot before I got pregnant but didn't sweat at all while I was pregnant and now, 11 months after giving birth I still don't sweat so it's definitely possibly that the smell associated with your odour has changed.