Tummy size changes.

I'm currently 17w 5d. Today i woke up and feel like my belly has gotten smaller compared to other days! Was a bit worried so used my doppler and baby heartbeat wasn't hiding near my pubic area. I had to move up higher this time and heartbeat was fine.. is it normal for belly size to go a bit smaller some days.. I just feel like I look smaller not that I have really looked big at all!