New Saddle Causing Skin Irritation

I recently upgraded my saddle from the Oval 300 that came on my bike to a Selle Italia Flow SL and I'm having an issue with it.

It's more comfortable on my ass and the cutout definitely helps keep blood circulating. I no longer feel the sensation of blood returning when I get up out of the saddle. This is the good news.

The bad news is that, for some reason, I'm getting a painful rashing on the bottom of my ass, where it curves back into the top of my legs. I feels like I'm chaffing and it's painful and stingy for a couple days afterwards.

I don't use chamois cream or body glide or anything but I'm wondering if I should start or if I need to return this saddle and look elsewhere. I love the look and the feel of the new saddle but I don't want this to keep happening.

For reference, I rode 50 miles over 3 hours last weekend for my first long ride in the saddle and about 35 miles over 2 hours today. During the week, I usually ride ~20 mile in-town rides and I guess it's just not enough saddle time to cause any issues.

I've done a few metrisc with my old saddle and I had numbness issues which is why I started looking for a replacement.

What do? Has this happened to anyone else?