It sounds to me like you are lactose intolerant and suceptible to the butter fat. Your usual symptoms sound very much like classic symptoms to lactose intolerance. It sounds like you need to become very strict with yourself and totally eliminate dairy from your diet, since you have had a history with it bothering your digestive tract and your sinuses. Totally classic lactose intolerance.
Since soy products aren't good for people, and many with food intolerances are also allergic to soy, I would suggest looking for a rice milk alternative. Most grocery stores carry a brand called Rice Dream. IF you're not sensitive or allergic to nuts, there is a milk substitute made from almonds that really tastes great called Almond Breeze. I can't drink it myself, as I am sensitive to nuts, including almonds. A lot of people with food sensitivities and allergies are allergic to nuts. This is why I suggested the rice milk substitute. If you have a Trader Joe's in your area, they have a rice milk substitute that they sell in the refrigerated section that they simply call Rice Drink. I like their brand the best and it doesn't have all those weird chemical preservatives in it that so many processed foods have.
If you continue to consume substances that you know you're allergic to, it is very possible that this will cause other health related issues. It certainly hasn't helped your digestive tract or you sinus allergies. It's best to avoid this possibility and avoid dairy products from here on out.
Well I called the place this morning and asked them about the milk shakes. I found out they use a lot of butter fat in their milkshakes... which is apparently not something done regularly since I've never had this issue before. What does it mean being allergic to this? I'm I possibly mildly lactose intolerant? I used to be an avid milk drinker and had stomach problems and they switched me to skim milk. I never had what I would call a noticeble issue after that. However, I have always found myself with a bit of a snotty nose (mostly drainage) what seems like year round and read that milk can affect your sinuses. Plus I moved and so I no longer drink milk in any quantity unless I visit home (skim milks taste oh so different between brands to me). I have other basic issues that wouldn't normally be contributed to allergies but is it possible that minimum quantities of something your allergic to over extended periods of time cause imbalances and other issues?