What this smell?

hi this may sound mingin but ive been gating this weird smell coming from my underwear wen goin to the toilet i only get it now and then.. its a very strong smell its not a horrible smell but very strong to the extent of nail varnish remover.. ive been checkd in the past and i havnt any infections or sdi"s and ive been with my partner 4 6years. ??? hope sum1 can help. xx