I have a little extra "fluff" around my mid section after baby. Does this shirt cling around the middle or is a straight, flowy fit?

I can sure relate, and I love how you call it “fluff”! I own several of these tops and they are awesome. They don’t cling to my fluff. I am 45, 41, I can’t recall my hip measurement and I’m 5’1” and the 2x fits me perfect with room to spare. Check the size chart and compare to your measurements. I’m a little larger than what the size chart says, but I’ve had good luck with these tops and I’m back for more! Enjoy!

No, this garment is not ribbed.

I want a fitted tank to where under tops in winter for warmth, like a girls underwear top. What size should I order? I'm 5'3", 113 lbs, 37" bust.

I'm guessing a medium or even a large. I'm 5'7" and 150 and 36" bust. a medium was tight. The length isn't the issue (5'7" vs 5'3") but the 36" / 37" is the issue. I thought it was too tight after washing (cold water) and drying (low). I like some freedom and didn't get any with it. Go bigger b/c cotton just continues to shrink a little more over time...

Does it have a loose fitting bottom?

Not necessarily loose fitting but it isn't skin tight. Id say it is a relaxed fit. The shirt runs a little big. It also stretches out the longer you wear it.