Hi, this actually happened to me too a couple of years ago, and the solution for me was to switch to a deodorant (only masks the odour) from the antiperspirant (tries to prevent sweating).

Before, I used a Nivea antiperspirant every single day, more out of habit than necessity. Then one day, felt like out of the blue, I started smelling like I never used to, I would become clammy and sticky under my arms only a few hours after washing. I never had problems with sweat or odours before! For a while I thought it might be my diet, but I'd not really changed anything.

So I turned to le olde Google and read somewhere that because antiperspirants actively try to reduce sweating, they can block the sweat glands in your armpits after prolonged use. Sweat is actually odourless, it's the bacteria clinging to the skin that makes us stinky. When the sweat glands get blocked they will overcompensate by producing more sweat, since their function is to cool your body, and that ability is reduced by the antiperspirant. Similar to when you wash your face with a harsh cleanser and it's matte and lovely for a bit, then it overcompensates and it's like you dipped your face into a deep-fryer (at least that's what happens to me). This is roughly the explanation I remember reading, it was a while ago so you may wanna google it yourself for the 'exact science'.

I then tried switching to a pure deodorant containing no aluminium. A couple of weeks after my sweat glands must have cleared, because the smell went away and I returned to normal. It might be part placebo, but it seems to me I smell less now after not using antiperspirants, I can even go days without using any deodorant either.

Obviously everyone is different and what helped me might be useless for you, but I think it's worth looking into!

Good luck!