Ay, possible talaga na baby mo yan kasi 13 weeks na siya at lumalaki na! Marami sa amin nag-feel ng ganyan around that time, parang little movements o bubbles lang, tapos biglang parang may something, then nawala. Pero since nag-disappear agad ang sensation at walang pain, most likely yan ay baby movement, baby's position shift, or kahit ligament stretching lang na normal sa pregnancy. To be totally sure though, better mention it sa OB mo sa next check-up kasi nandyan na siya to rule out anything; ang peace of mind mo mas importante!