Dark underarms?!

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My under arms have gotten really dark over the past couple weeks. I figured it was hormone related but now I'm experiencing a mild rash and my lymph nodes feel swollen and a bit sore. I can't figure out what is causing it because I've 1. switched to a natural deodorant two weeks ago 2. had the tDap 3 days ago which my doc said can cause swollen lymph nodes. Anyone else have armpit issues?!