Well certainly our bodies change as we age, I don't know that it necessarly has to do with having babies. I can only speak from a personal standpoint on that... when I hit 30......what can I say, I felt like "who took over my body and where did my real body go"? Don't know if anyone else out there has felt this or not.
Regarding the rash, my guess would be that when you have kids, THEY expose you to many more virus and bacteria with all the stuff they bring home from day care, school, etc. IMO. That's the mom in me talking. Been there, done that
It is not uncommon for rashes to get worse after a warm bath or shower. It is due to *vasodilitation* (dilation) of the blood vessels, that is what causes the flushing, when you cool down the flair of reddness should get beter.
Hope you feel better soon.
D