Drink cold water and relax for 15-30 minutes.. If you still don't feel much activity in that time frame I would speak to your doctor.
I actually had the same thing around 25wks, I'm 34wks now. I was so worried, but I still felt a twinge here & there, my OB told me around that stage he/she goes through a growth spurt and they slow down for a few days. He soon perked back up :)
The baby may be toward your back more at the moment and depending on where the placenta is could have an affect on feeling his movements. My little boy moved a lot even for 18 weeks you could feel and see where he was but I'm currently 21 weeks and some days I barely feel my little girl but my little boy is currently 10 mths now so I could be more active and that will make a big difference.
I had that concern once I didn't feel him for 3 days so I called my doctor and they told me to drink something really cold lay on my back and wait 30 mins it worked :)
I wouldn't too much worry especially if he"s moving softly but at your next doctors appointment ask your doctor but pretty sure it's nothing