My kitty stopped eating!

A few days ago, Strawberry got sick. He threw up his breakfast and then after that, he just didn't feel well at all.  He wouldn't eat the rest of the day. The next morning he licked some juice from some canned food, but nothing else. When he wouldn't even eat his snacks, I knew something was wrong. He never turns down a snack! Also I noticed his belly was really swollen, but it wasn't hard and he didn't seem like it was painful if I mashed it, he didn't have a fever. I called the vet's office and she was already gone, so I made an appointment for the next morning. Later that night, my husband called me at my studio to say he was throwing up again. I called the vet and she had me give him some hairball remedy (in case it was a hairball and to give him a little nutrients since he wasn't eating. I worried myself to death over him all night! He did eat just a little the next morning, but I took him on to the vet. The problem was he was very constipated. The vet said they won't eat if they are that constipated. She gave him some Lactulose before we left and sent me home with a bottle. So, $71 later (yes, that lactulose cost almost $50!), Strawberry was already feeling better by the time we got home. A couple of days later, and he is feeling good again. I will definitely be holding on to that liquid gold just in case of emergencies! I don't guess I realized how sick being constipated can make a kitty!