It sounds like you little one is just getting over a stomach virus. Unfortunately, there is not a lot you can do about stomach pain from viruses. Mylanta and other antacids are primarily for gas or gastric juice problems and not really for the cramping that causes the pain post virus.
I have found eating helps. This can be a catch 22. Sometimes it causes the vomiting again, but sometimes it just gives the stomach something to work on, so it stops cramping. I would do bread or dry cereal.
One thing to really be careful of is dehydration. Kiddos can get dehydrated really fast, especially with vomiting, and this can cause more serious problems including stomach cramping and diarrhea (which is often mis-construed as part of the virus). Gatorade replenishing or electrolyte is best (my kids hate pedialyte) as is some watered down gingerale...but the best is plain old water. Good luck and I hope we could help!