stomach gets a little tense

Anastasia
I have not read about this before so it does catch me by surprise. 
Since week 16 I have have felt the movement of the baby. I am now 21 weeks. The movements that I feel honestly feel the same as what I felt in week 16 just a little more frequent. Even still they are rare and light movements. 
One thing that I do notice is that my stomach will start to feel a little tense and I'll put my hand where it feels tense and it is super hard. It's usually only a certain area but I'm assuming that is where the baby is laying. The tense feeling will fade and so will the hard area. Interestingly enough it is never accociated with movements. 
Has anyone else had this? It's pretty cool