Eh, kena check dengan doctor la cepat-cepat untuk confirm apa exactly, bisa ulser, bisa thrush (fungal infection), atau benda lain je yang perlu proper treatment! Since dia minum susu okay, it's not urgent-urgent but jangan delay sebab area bawah lidah tu sensitive especially kat babies. A few of my mum friends had similar and most turned out kena thrush from milk residue, which needs antifungal cream from the doctor. Better safe than sorry, ya?