Hai dear, at 28 weeks the tightening sensations are pretty normal, your uterus is getting ready and practising, so don't worry lah. But since it's been happening for 2 days non-stop, it's best to give your doctor a quick call just to be safe and get peace of mind, especially if it comes with any pain, spotting, or you're feeling unwell. Make sure you're resting well, drinking plenty of water, and taking things easy, sometimes dehydration or being too active can trigger more of these sensations. You're doing great, mama!