I have a rash under both of my breasts. I have very large breasts. The rash is very pink/red and itchy. I have been trying to keep it dry and have applied monistat cream as well as calamine. Is there anything more specific I should be doing?

Its just like diaper rash... and can be treated the same... with the same products. Prolonged moisture exposure and friction... happens to people with very large thighs (skin on skin means sweat pooling and remaining wet for prolonged periods)... cyclists would ride in the rain for prolonged periods and even people who are in very hot environments if they wear clothing or have wet skin for prolonged periods.

Wear natural materials that allow moisture to wick away better....along with the right ointments....it could go away in as few as a couple days.

Yes it is uncomfortable and can be outright painful.....I had a similar rash on a long motorcycle trip where I was wet from rain for several days, most of the day.

If you can, don't wear a bra around the house, but do wear a fitted cotton top with lycra/spandex in it, so that your breasts are not against the skin below.
Don't use Monistat unless you know it's yeast, which I doubt it, or you would have a very smelly ooze on the skin (intertrigo). Calamine isn't quite right either. Get some cortisone cream, and apply after gently washing with unscented soap and lots of rinsing.
Most of the problem is getting air around the area. Lie back a few times a day and lift your breasts, and pat a little cortisone cream on the rash, but not a lot. You want air there too.
Stay away from underwire bras if you can afford some bras with a wide band underneath instead.