I had a cow develop something very similar last summer. She was a nine year old that had calved in March and I started to notice some swelling on her belly/side just about weaning time in early August. She was AI'd twice in May and did not stick, so was in the bull pasture until winter. She just calved about 10 days ago with no problems. When I first noticed the swelling I caught her up and had the vet look at it right away. It looked very similar to your pictures but was maybe just a little more on the side and a little less on the underneath of the cow. Vet poked around and said it was an abcess. He couldn't say what might have caused it. He said let it go and it will come to a head and burst. Well, a couple weeks later it was twice the size (like twice the size of a basketball hanging off there) so he lanced it and got a ton of real nasty gunk out, cleaned it good, put her on low dose penicillin schedule that I admministered, and now you can hardly tell that it ever existed. She'll probably find the cull pen at weaning time this year due to age and late calving, but she seems ok other than that. Also had one of our best young cows start to do the exact same thing this winter, but it only got like fist sized, burst, and healed without my intervention.
From all the replies, it could be a number of things, ranging from harmless to deadly. I'd get the vet's opinion before shipping a heavy bred cow just about ready to pay her bill for the year.