Are these IBS symptoms?

While some people with IBS also have stomach problems, I don't think I've ever heard of IBS being nothing BUT stomach problems. If you aren't also having bowel problems of some sort, then I think your doctor was getting too free and easy with the IBS label and you need a second opinion. In fact, I think the Rome Criteria used for diagnosing IBS says you have to have something wrong with your intestines/bowel movements.

Has anyone checked you for food allergies? Food allergies can cause the upper GI problems you describe, especially the excess mucus If you don't want to wait for a doctor, you can try and discover them on your own: eliminate for three weeks, each of the following (only eliminate one at a time):

Milk (this is especially suspect if you have any Asian or African ancestry, as they are the most intolerant to cow's milk)
Gluten (found in wheats, etc. grasses)
Seafood
Nuts
Sugar (artificial and real; limit fruits, no fruit juices)

If you notice an improvement in your health, then you might be allergic to that product. You may want to get tested to confirm, but some people are not clincially allergic to something, but have a severe intolerance for it anyways. That's why it can help to do an elimination diet as opposed to relying just on allergy tests, because the allergy tests themselves aren't always accurate (although you're more likely to get a false positive than a false negative), and they don't gauge a non-allergic intolerance to something. I'd probably not come up as allergic to sugar alcohol, but even the tiniest bit of it kills me. I tell people I'm allergic to it just to make sure it doesn't get put into any of my medicines because I'd be sick as a dog for days if I got into it.

And have you had your white count checked to make sure you don't have some sort of lingering cold/infection? That's another cause of mucus and the tightness in the chest. I once knew a guy (older guy) who went down at an airport with what everyone was sure was a heart attack (he was terribly obese). They cracked him open, sure he needed emergency bypass, and they found that his heart was fine but the lining around it was massively infected. I don't know if they ever found out why he had an infection around his heart, but infections can masquerade as other problems. A lady at work was just out with what seemed like gall bladder problems, but it was an infection in her pancreas instead.

Have you ever considered that it might be acid reflux?

Yes, allergies can seem to develop suddenly. Although you may have been growing increasingly intolerant of something for a while now and just haven't noticed the symtoms or chalked them up to something else.

I think a food allergy could cause tightness in the chest and shortness of breath because it can inflame the throat and esophagus, and either of those can cause the problems you are experiencing.

Also, try taking some probiotics. If you decide you can tolerate milk, you can eat a yogurt a day (we all here like Activia or Stoneyfield), or you can take a probiotic pill, like Digestive Advantage. You can find it in the bowel products section of any chain pharmacy. Your antibiotics may have killed off just enough of your stomach/intestinal bacteria that you are having some problems. Sometimes a bacteria imbalance can cause excess mucus production. Probiotics' only side effect is gas, and that usually dissipates after a week or two on them, so it never hurts to try them for 3-4 weeks and see if you get any better.