Sore belly :(

Myriam
I am in my 31st week, and yesterday at lunch I started getting some tightening in my lower belly. It was followed by light on and off period like cramps. 
This morning I woke up with the right side of my belly feeling really sore like after doing an excercises. The pain comes and goes, sometimes feeling ovulation pain, and sometimes like gas. When should I be concerned? This is my first pregnancy so j have NO IDEA what's normal and what's not. Thank you so much mamas.