L.R.
Poor kid, it's hard being sweaty and in a diaper! She might be having a bit of a contact irritation or reaction between her sweat and the plastic material of the diaper.
I avoid baby powder and only use cornstarch -- it can't harm anything (baby powder is the mineral talcum, which people shouldn't inhale or get in their mouths). Try ensuring she's very dry and then wiping on some cornstarch with your fingertips rather than "dusting" with it. No powder will last all night, probably, even the cornstarch, but wiping it on rather than dusting should help keep it on a bit longer.
You could also consider changing her brand or style of diaper, in case it's the waistband of this diaper style that's an issue. How tight do you keep her diapers? Is it possible that to keep them from leaking in bed you're putting them on pretty tight which is making her sweat more around the waistband and on the backside?
Does she tend to itch there during the day? Do you see these bumps only in that area and only at night when she's sweaty or do they turn up during the day too -- and if they show up in daytime are they present only when she's sweaty in that spot as well?
Also consider -- this started in recent weeks. Has it been getting hotter lately in your area as summer approaches, so maybe her room's warmer at night than you realize? Does she love to snuggle under blankets to sleep? She might need a readjustment for her room temperature and might need a lighter blanket or lighter pajamas too, to help with the sweating.