I am not sure, I would get it checked by a doctor.
I occasionally have the same thing. I have larger breasts and it's usually under and inbetween them and especially if I sweat or like after I've been sleeping. It never happens after I shower but if I miss a shower or like I said I sweat or it's hot out or something when I come to bed at night and take off my bra I smell it a little bit. It doesn't smell bad, like you said it smells sweet. I'd like to know what it is as well.
It's yeast and as long as you stay clean you won't get a rash or anything. It just builds up or grows in moist places. It's normal.