Girls, my little one (we're 3 weeks old) started rashes about 5-6 days…

Girls, my little one (we're 3 weeks old) started rashes about 5-6 days ago, similar to diaper rash, there was a bit on their face and tummy (1 photo), according to advice I started treating with chamomile infusion, but it got worse, they became redder and much bigger ? Do you think what it is, and how do we treat it? Or should we just let it go?