Interesting that it is just 1 armpit. Seems like if it was hormones or diet it would affect both armpits. I have had 3 kids and didn't experience this thankfully. I am like you - never really NEEDED deodorant. Didn't ever really smell unless I had a super stressful unexpected incident. I DID experience a change of body odor while I was studying abroad in Germany - had to be on antibiotics and smelled different to myself for weeks afterward. I felt like I had been colonized by a different bacteria. Eventually it went away.
With it only being 1 armpit, I would wonder if you had a bacterial or fungal infection or just got colonized by a different "normal flora" in that armpit when your chemistry changed with pregnancy. It may be something as simple as using antibacterial soap only on that armpit in the hopes that the "normal flora" of your other armpit would colonize that armpit as well. The bacteria live on the hair shaft and follicle so I would guess that shaving DAILY that armpit and using the antibacterial soap on that armpit daily for a while may increase your chance of a change in flora. I would put deodorant on the "good" side first.
Not to freak you out but - there have been cases of dogs smelling breast cancer before it has been detected otherwise. There is mammary gland tissue in parts of your armpit. Have you done a self exam lately? Always a good idea to keep that up - just a reminder.
Let us know if you find anything that works.
Best wishes!