A: 17 months is a critical developmental time because from 18-24 months language is developing. Figuring out the source of the diaper rash would be great because so much needs to happen in the coming months. Based on my experience with my son and the many doctors we consulted about his rashes and food sensitivities, I would suggest eliminating all casein (dairy), gluten (wheat, rye, barley and oats) and soy for a while to see if the bumps go away. After the rash goes away, you can add them back one at a time to try to figure out which one might be causing the diaper rash. Because the molecules are so similar, it is sometimes easier to eliminate all of them and add them back to try to get to the bottom of a sensitivity. Although it might seem drastic to eliminate gluten, dairy and soy of for a few weeks, finding out any sensitivities will be beneficial in the long run. You mentioned that your daughteris drinking soy milk. Many children who are sensitive to dairy are also sensitive to soy, so I wouldn't recommend soy milk to anyone as a replacement for dairy. In fact, I wouldn't recommend soy milk to anyone. The benefits are soy, in my opinion are largely a good public relations campaign by the soy industry and only present if you are consuming fermented soy in the context of an asian diet. Weston Price has a good summary about the dangers of soy. Also, in Soy Formula - Better than Breastmilk? Weston Price's has this to say about soy for babies:
The most serious problem with soy formula is the presence of phytoestrogens or isoflavones. While many claims have been made about the health benefits of these estrogen-like compounds, animal studies indicate that they are powerful endocrine disrupters that alter growth patterns and cause sterility. Toxicologists estimate that an infant exclusively fed soy formula receives the estrogenic equivalent of at least five birth control pills per day. By contrast, almost no phytoestrogens have been detected in dairy-based infant formula or in human milk, even when the mother consumes soy products.
When you add items back you could first add some wheat items that don't contain gluten like barley. If your daughter reacts to barley, then she is sensitive to gluten. If she doesn't react to barley, but reacts to wheat, then she is sensitive or allergic to wheat. My son was super sensitive to traces of barley contained in Rice Dream milk, that is how we figured out my son's gluten intolerance. Here post describing a yeast infection diaper rash that might be of interest.