Hay, those tummy tightening sensations you're having sound like Braxton-Hicks contractions, which are super common starting around second/third trimester, a lot of my mum friends experienced the same thing around your weeks! Pero since you're already at 30 weeks at nagiging frequent na siya, better to mention it sa OB mo next check-up para ma-rule out anything and para ma-ease your mind, lalo na kung may pain or spotting along with it. Hugs, pregnancy worries are real but your doctor can give you the peace of mind you need!