Missy,
It sounds like you are really expereincing a difficult time right now, but I think seeing your Ob next week is a good thing, possibly they can shed some light on what is happening for you. It is still a tender stage at 16 weeks so do what you can to take it easy. I kinda agree with Lenny, the pains may be stretching and ligament pain. Do a search on google for 'stabbing pains' during pregnancy (if you haven't already) and it will show up that it is quite common in the early stages - there's heaps of info on it.

I don't know of any vit's that might help, but as they say, the healthier your diet, the less your body will actually need supplements, so go hard on the fruit and veg, servings of fish and red meat etc. and you'll feel better for it.

Also, you mentioned knowing when somethings just not feeling right, and most times I agree, we just know, but there are times when we're tired, anxious and emotionally drained that we can get mixed messages from our bodies and when our mind starts to play tricks on us or puts doubts out there. I hope this is one of those times for you. Oh, and I know it probably wont help but try to remember that your body has done this for you before with the 2 beautiful children you have as proof. Hold onto that while you await your Ob appointment next week.
All the best chicken.