question about stomach muscle pain and changes

I'm a very petite girl and have always had definition in my abs (not a six pack, but everything tight and fairly defined). For as long as I can remember, I've had very minimal fat on them. On Friday, I went with my younger sister to a workout with her personal trainer and we did the most intense ab workout I've ever done. The following day, Saturday, I was in so much pain I could barely move, and what's more, my abs were unnaturally defined, making it look as though I had gotten a six pack over night, which I know is impossible. They also really, really hurt to the touch and even the slightest pressure was extremely painful. However, yesterday, all the definition was completely gone and I looked as though I had gained about five pounds of fat, all of which is sitting very unflatteringly on my stomach. I was still in a good deal of pain yesterday. Today, I'm not as sore, but moreso very confused as to why my entire physique has changed because of one workout--Ive gone from my usual fit and petite frame, to unnaturally muscular, to the appearance of having gained a lot of fat--all since Friday. Can anyone out there explain what's going on? It really appears as though my abs are swollen, but as the pain goes down, the swelling is not. Help please!