Basically too much of anything that is hard for your body to break down causes inflammation & joint pain is a common symptom of an inflammatory response in the body. These symptoms can feel amplified in your beat up joints.
Bristow is right about the liver, if it's not happy you/your body won't feel happy. Food, alcohol, drugs/meds, etc are largely broken down by the liver. The harder your liver works to break shit down the more internal inflammation. At you age it should just be temporary effects of doing it up too hard from time to time, but things do add up.
Also, when you are really drunk you may pass out but you don't get real rest/sleep generally. When you sleep shitty your body does not heal itself as well or quickly as usual. So I guess too too many beers + Bmx = sore joints