Why does my 1yr old dachshund shake when he smells my other dogs pee?

This just started about 3 weeks ago. i have a 1 yr old dachshund (monster) and a 3 month old chihuahua (dolly). When i take them out to pee monster will usually go burry his nose in dollys pee for a few like 30 seconds and while doing that his head starts to shake alittle and his teeth start to shatter... it is odd because dolly is too small to be in heat so its not like monster gets excited over that issue. so what else can it be? hes a healthy dog other than this weird thing he started