It's normal to have less movement around 18weeks. Baby most likely moved and its hard to feel baby kicking etc.
Since the baby has a good strong heartbeat but you aren't feeling movement, it's most likely moving and you just aren't feeling it.
Sounds like he/she has moved to a different place where you are less sensitive to movement.
I felt the baby before i got sick. Im 18 weeks and find out the sex in two weeks
Im sorry i just seen how many weeks you were your baby is not going to move that much until your like 28 weeks n further