Eh, that vibrating feeling is likely hiccups, dear! A few of us mummies noticed the same thing around that stage, it's like a rhythmic flutter or vibration yang boleh quite strong actually, especially if baby is lower in your belly. It's totally normal and nothing to worry about, but since you're a first-time mum and want to be sure everything's good, best to mention it to your doctor at your next checkup just to put your mind at ease.