18 weeks Pulsating belly/uterus?

Josie • Cosmetologist ✂ Nature lover🌲🌻 NLB♡ Matix Roy💙Sienna Colleen🌸 & baby due May 20th
Im 18 weeks, and sometimes when I put my hand on my stomach pulsating or something, just curious as to what that is. It doesnt feel like baby moving. Just an organ? But wondering if anyone knows what it is?