Well, I think I know the problem and hopefully solved it before knowing I did
My saddle is narrow, and too make things worse, I recently switched to one of our other saddle pads, which is very thick. This caused my saddle to not fit correctly and even the slightest weight adjustment caused my saddle to slip even though the cinch wasn't lose. Therefore, I had to tighten my cinch tighter than usual (not painfully tight, just tighter than she is used to). So today, before I noticed the 'swelling' on her stomach, we were at the tack store and I got a much thinner pad after diagnosing the problem of the slipping saddle while talking to a girl who worked there! :) I hope this is the only thing that caused it! I will continue to watch the swelling and massage it if need be
Let's hope the new pad works!