It looks to me like she has swelling, or edema, on the floor of her belly. Your description of a fingerprint lasting 20 seconds or so before filling in supports that idea. This is not uncommon in pregnant mares, especially if they are confined or not moving around much. If you can increase her excercise by turnout, handwalking, or even putting her food and her water on opposite ends of a corral so she has to walk back and forth more than she is, it might help. If it doesnt improve, or it gets worse, your vet should see her to rule out circulation problems.
Here is some more info on edema in pregnant mares-- http://www.thoroughbredtimes.com/hor...ant-mares.aspx