Are they warm to the touch? Whn horses develope absesses they start off warm and hard and get soft as they prepare to burst. Haematomas are usually soft, and they are caused by broken blood vessels. Theyre more common in the brisket area however and I doubt you'd find two perfectly matched. Swelling under the belly can also be caused by congestive heart failure. When the heart is pumping blood out into the body. You would have to have an ultra ound done on the lungs to determine this. I hope your horse will be ok.