I am 35 weeks 4 days. stomach keeps tightening and releasing. when should i be concerned?

If they dont stop: It is normal to have irregular contractions off and on as your uterus prepares for labor. If they contractions do not stop with hydration and rest and if they are coming at regular intervals every 5-10 minutes then you should go to labor and delivery to be evaluated for preterm labor. Because 35 weeks is still preterm it is better to be safe and go for evaluation.

Braxton Hicks: This sounds like Braxton-Hicks contractions, also known as "practice" contractions. This is your uterus contracting and then relaxing. It is normal and not concerning unless the contractions are painful - then that means you are in labor. Sometimes these can happen if a person is tired or dehydrated, so make sure you rest and drink some water!