Katz, swollen lymph nodes are usually a sign of an infection somewhere. Mine were horribly swollen and painful when I had strep throat. Strange, though, that your blood work wouldn't show an infection unless you developed it afterwards. Or it is possible that you showed evidence of a mild infection that they didn't think was enough to bother telling you about. Some doctors don't bother worrying about very low-grade infections since a lot of people have those frequently, especially during allergy season. People can be sick a lot of times and not know it because our bodies are so good at fighting that stuff off. We really only notice when we're losing.
I don't know why they thought your nausea was IBS since only some IBSers have nausea and as far as I can tell, that's in connection with pain, gas, diarrhea and/or constipation. I don't think I've heard of IBS consisting of nothing but nausea.
Things that can cause nothing but nausea are a hietal (sp?) hernia which is where the stomach pokes up a little into the esophagus or the esophagus down into the stomach--one of the two (or maybe both). Anways, a slight connection problem there. I don't know if they will find it on an ultrasound; they found mine by enoscopy (mine is not so bad as to cause me problems, though--seems a lot of people have them and never know about them). Also acid reflux or excess acid in the stomach can cause nausea. You don't necessarily have to have heartburn since the acid may only reflux part of the way up or stay bubbling in your stomach and not cause the distinctive burning sensation.
Your gall bladder can also cause nausea, although it is usually accompanied by pain, vomiting and/or diarrhea. But not all gall bladder symptoms come on at once, so you may develop these later. Early into my bad gall bladder everything I ate made me feel sick to my stomach, so I quit eating much and lost 20-30 pounds in about 5 months. I got very weak, was tired a lot, couldn't concentrate, and generally was lethargic. I had one or two bad pain attacks during that time and I think I had some diarrhea too, but the nausea was the worst. I eventually got to where I had less nausea and more diarrhea and pain. (That wasn't exactly a fair trade-off.)
They may find a problem with your gall bladder on an ultrasound, but only if it has stones, sludge or is deformed. If it's chronically inflamed like mine was (may explain those swollen lymph nodes), they can pnly discover that by doing a hida scan, which checks for gall bladder function. If it doesn't function well, there's your problem.