We use QV gentle bath wash, Dermaveen moisturising lotion which can both be purchased from the chemist. I had to take her to the dr a few times to get medicated creams during her first summer as it flared up so bad I could not get it under control.
When my daughter was younger I found I had to avoid putting her in bonds wondersuits as it always got worse around her neck if she wore them, no idea why but it was really obvious it would aggravate it heaps. I made a big effort to dress her in only comfortable cotton clothing when it was particularly bad.
Only put a very small amount of washing powder in the washing machine if you really have to, but as others suggested I would probably try washing without any powder or every second wash and when you do use only a tiny amount and definitely not the recommended amount.
If you are breastfeeding look at your own diet. Are you eating anything new, having a lot of dairy?