Aw, that's worrying talaga mama! Honestly, bumps sa likod ng tenga can be a lot of things, sometimes it's just a harmless cyst or lymph node na swollen dahil sa minor infection, pero kailangan talaga i-check ng doctor para ma-rule out ang anything serious, especially sa ganyan kalaki na 3 months na lang ang baby. A few of us moms had similar concerns at dinala agad sa pedia, kaya better safe than sorry, hindi mo dapat i-wait out kung concerned ka na, lalo na sa ganyan kalaki na bukol!