Can you please explain why the lymph nodes in my neck swell and then shrink?

Infection: The lymph nodes drain inflammatory fluids away from sites of infection. If swollen node is shrinking it is doing its job, if it continually swells and shrinks, you likely have a persistent infection, an example would be a dental abscess that drains and gets worse with time, but can reduce temporarily with a route for drainage through tooth or gums. See an md or dentist and good luck.