I used plain white flour to make homemade bread. I followed the recipe from the 2011 cookbook titled "Blue Ribbon Recipes". I have been using this recipe since I bought the book in 2011, and have never had my bread smell like sourdough before. The smell was strong enough that it overpowered the smell of homemade huckleberry jam. I store my yeast in the refrigerator. There is no vinegar used in the recipe, and the yeast starter sat for approximately 10 minutes while I measured and added the other ingredients to the bowl. I've never had this happen before.
It could be an issue with how long you let it rise or maybe how warm you rested it at. I was making a deep dish pizza at home once and the recipe called for resting the dough in the fridge (mixed with the yeast and other ingredients), for 4hrs minimum. Due to my getting home a little after 5pm from work and some of my family leaving for work at 6:30pm, during the time I was attempting to make this, I couldn't use the suggested times for the different steps of the recipe. To shorten that time, I made the dough the day before. I let the dough rest in the fridge from about 9pm until 5:30pm the next day. When I made the pizzas the dough was very good, but it tasted a lot like sourdough, and that was using instant yeast out of a packet. I've since made the recipe with the correct resting times and it's good dough, but there is no sour taste.
So a long resting time for your bread, or possibly having it warmer might induce the yeast to process their 'food' faster. I know it's not true sourdough as the sourness there comes from a combination of yeast and lactobacilli's, but I did get a similar taste from the instant yeast packet and it was because of the duration I let the yeast work. Some of my family actually prefers the sourdough taste of that dough, so I sometimes make it that way. It also saves me from having to do too much work on any given day.
You can test my theory out and see if temp or the leavening time affects the taste. While you didn't originally want that particular flavor, you may come to enjoy it.
When bread is fresh it may still be saturated with carbon dioxide which will give it a somewhat sour-ish smell.