GERD and IBS or what?

I have had longstanding GERD for 15 years now. I started having upper endoscopies 7 years ago during which I have had 5 upper scopes. The first four showed that I have GERD grade A and a competent sphincter. The last scope done last February showed GERD grade B and an incompetent sphincter. I have no idea what's the cause of these changes. I was put on Baclofen 10 x 3 along w Nexuim 40 mg double dose and famotidine 20 x 3.
Lately, whenever I drink more liquids (I normally drink 3 cups of diluted tea w drops of milk or homemade juice) or have an extra bite more than what I get everyday (5 small snacks); for example, when I go out w friends. I immediately feel nauseated and can't have a sip of water to take medicine, severe sinus headache starts and d hits. I can't find the connection. But I feel that my LES sphincter is very loose and I can't hold food or liquids down.
My GI was travelling and I'll try to see him and bring that to his attention. I'm just thinking: is it GERD that is causing this? Or is it pressure on my colon that causes pressure on my stomach and pushes stomach acidity upwards?

Kitt, thank you very much for this informative post. I really appreciate it. Still I can't figure out the relationship between LPR and my colon. I woke up w gurgling noises, had a burning sensation in my colon and soon d hit. I was straining when I had a bm.I think my proctitis is acting up. I have suffered before from narrowing of the rectum. I think it might be the case this time. Will try to see my GI soon.
Take care :)