I think whether you have stretch marks are down to genetics. The anti stretch mark creams help with skin elasticity, but it's not magic. If you're genetically more prone, you might still get them even after slathering the cream. Still good to apply even after birth though, since the skin will contract. There are treatments to reduce stretch marks (e.g., creams with retinol and laser), but mostly can only do after birth. If you don't feel any pain, the tummy tightening is likely Braxton-Hicks. It's normal. I'm also at Week 38, and I feel this happening more often also. My gynae says the Braxton-Hicks is to help prepare the uterus for the actual contraction, and it helps to push the baby into position.
it's normal you need to spam more stretchmark cream to prevent more from happening.