My bet is excitement babys bones would be as soft as spaghetti if it even has many bones at all so its definitely not baby
Its most likely gas. You can't feel the baby at this point. You usually cant see it on a sonogram until 6 weeks, even with a internal one. Had one at 5 weeks saw nothing had one at 6 w 2 d and had a little bblob inside me! Your baby is about 2-3 cm big, if that lol.
This is my first and I started feeling my baby kick at 16 weeks. Fluttering at 15. I was underweight when I got pregnant, my ob said that's possibly why I felt it so early. Every body is different.
Although, 5 weeks is wayyy to early. You are most likely feeling gas.
it's probably your uterus stretching, you can feel the baby move around 20 weeks, earlier if this is your second
Since I found out I were pregnant I too felt a buttery fly flutter feeling in my stomach especially when I lay in bed thinking about being pregnant. I honestly think its just excitement/nerves like you get on Christmas eve as a child.. :)