I just had a few beers that have been sitting on the shelf for about 3 months and then in the fridge for several days. All three of these beers (pale ale) were very foamy, not gushers but a very large head. they were not like that at even 2 months old. They tasted fine. Any ideas as what the cause would be so I can prevent this in the future. This was my third brew. thanks in advance.