This sounds like good old MS to me. My MS was so bad I regularly found myself heaving nothing but bile (that green / yellow stuff). I found that I got the pain in my abdomen to. this'll sound bad but try getting down closer to the loo if you're at home. I found to avoid the pain completely I had to kneel next to the loo. It's your tummy muscles working so hard to evict nothing.

But trust me, there's no such thing as over reacting because to much MS can lead to needing a stay in hospital getting rehydrated!! And that is something no-one wants.