belly feeling smaller at 15+4?

Twin mamas! I've been sporting a pretty decent bump the last couple of weeks. Obviously bigger and harder at end of day, but this week I feel like I'm losing weight! My belly doesn't feel quite as firm or round (until late in the day) and I don't feel like I'm filling out my shirt like I was just a few days ago... I've checked heartbeats twice because I'm weirded out by it 🙄 and everything seems fine.. is this normal? Like maybe babies are further in my back this week? Thanks for advice! FTM and nervous about every little thing 🙄😂