Lactaid

Hi all! I'm having a bit of a problem. I'm 2 years out from surgery and suddenly feel like I've been socked in the stomach all the way through to my back. This happened last November and after a trip to the ER determined that I'd just over ate, got something stuck, and in a day or two I was fine. About 2 weeks ago I got the same feeling, thought I'd overdone it again (but couldn't determine what was the cause - I'd thought I got a pill stuck). It got better after a day or two but then a few days ago it started again!! Now it's been about 4 days and I'm miserable. I can still eat/drink/use the restroom, have no fever, vomiting or nausea, but my stomach feels so unbelievably tight! I've been scouring my brain for a common denominator and after drinking a protein shake it hit me, I've only recently started using Lactaid fat-free milk (instead of the Silk soy milk I've used in the past). Has anyone had an adverse reaction to Lactaid??? I know this isn't anything warranting a trip to the ER mostly because I don't have insurance anymore and I have no other symptoms to say it's gallbladder or pancreatitis or a blockage. I just can't figure it out! Anyone??