Monistat and Chamois Butt'r have 2 different purposes. The Butt'r is for lubrication, to reduce friction. I would not recommend putting it on your girly parts; there are lots of threads about this.. I put it on my rear or in the crease of my legs. However, i have replaced it with Body Glide.
Monistat is for curing yeast infections. Are you talking about the cream that you can use on the "outside?" I use that AFTER a ride, when i am freshly showered, only if I am having a particular problem. You shouldn't be using the Monistat that is used internally every time you ride!
In the spring, I have sometimes have this issue. I get a UTI from riding a lot, ramping up my miles. Then, the cure for that gives me a yeast infection. Last year I had a BVI (the worst), too.
So, this year, I upped my yogurt intake in March. I had to use the external Monistat cream for a week or so; I thought i had an infection, but I went to the doc and it was just "itching." I am hyper vigilant about getting right out of my shorts; I ride with a group once a week and I always bring shorts to change into if there is a lunch at the end. No one else does, which surprises me. I wash my shorts in hot water and hang to dry. Things have been much better.
And I second the suggestion that it might be hormonally based.