About a month ago, I noticed one or two brown spots in my underarms. They didnt cause any problemsno itching or irritationbut now there are three or four, and its starting to worry me. My dad had something similar, and it eventually spread across his entire underarm area. Could you help me understand what might be causing this and how to prevent it from getting worse?

That could be P versicolor you have to maintain personal hygiene and special care towards the affected area,visit your dermatologist for appropriate treatment.