As the above poster mentioned, it is normal for the heart rage to increase after a meal, that is part of the digestion process. As to the 160bpm, that is high, if that keeps up, I would have a cardiologist look into the matter. You might have some sort of food allergy as well, I know in folks that have food allergies, their heart rate will really pick up when they eat the offending foods.
Eating makes everyone's heart beat faster. It's needed to send more blood into the GI tract for absorption and to move the muscles that move the food around.
Some people experience higher heart rates for some reason. I've never seen anyone come up with a solid explanation. If you want, you could see your doctor to get a workup just to be sure. 160 is high just for eating though.
As for the buttermilk, are you sure you aren't lactose intolerant? I figured out I was lactose intolerant on my own after 10 years of chronic issues that couldn't be explained. It turns out that ~75% of adults show some degree of lactose intolernace into adulthood. Nature didn't intend us to drink milk past childhood I guess.