At 17 weeks, this kind of painless tightening is usually too early to be true Braxton Hicks, which shows up more in later pregnancy. Since you have no bleeding or pain, it more likely points to round ligament stretching or normal uterine tightening. On its own, this isn't how preterm labour usually presents.
Next Steps
Keep track of how often it happens and whether it ever becomes painful or regular. Staying hydrated and avoiding standing for long stretches can ease round ligament discomfort. Mention this at your next prenatal visit so your obstetrician can confirm with an exam.
Health Tips
See your obstetrician the same day if the tightening turns regular and painful, or if you notice bleeding, fluid leakage, or pelvic pressure that doesn't ease with rest.