Wa'alaikumassalam! Aa normal je tu actually, around 19-20 weeks, your baby's muscles are developing and you'll start feeling those hardening sensations, especially kalau active baby ka. A few of my mum friends also felt the same thing at that stage and doctor kata it's all part of the normal pregnancy development, tapi sebab ni first time you, mesti rasa macam something different kan? Definitely mention it to your doctor during next checkup just untuk peace of mind, especially if it's accompanied by pain atau discomfort, then doctor can check properly. Congrats on your pregnancy!