on and off dull ache in tummy 18 weeks pregnant

I normally wake up to a smaller and softer tummy everyday. Today, I woke up and my tummy was almost as big as it was last night and it is a bit hard.
By noon, I noticed that I feel some dull pain between my belly button and my pubic bone that would come and go. On a scale of 1-10 it is just 1 or maybe even lesser. Should I be worried? 
Other than that, I feel great, no bleeding, fatigue, vomitting etc.
Also,  could it be gas? I ate a lot of watermelon before I went to sleep last night.
Update:
OB told us to go to L&D just to be sure. Nurse told us it was because of fibroids. She said she did not see any contractions which I wasn't sure because I saw some spike in the graph. I was told to go home and just continue with the regular checkup schedule.

Glow Resources

Let’s Glow

Glow is here for you on your path to pregnancy

Glow helps you navigate your fertility journey with smart tools, personalized insights, and guidance from medical experts who understand what matters most.

25+ million

Users

4.8 stars

200k+ app ratings

20+

Medical advisors