Stretch marks are due to ripping of collagen in the bottom layer of the skin. This happened to you because your skin was not strong enough to handle being stretched that much at one time. They show up dark or red at first because the body is growing new blood vessels to help it heal.
The good news is that the blood vessel growth stops after a couple months, and the skin color usually lightens a lot on its own. Your best option is to wait at least 6-12 months for your body to improve the color on its own. Using cocoa butter will help soothe the skin, but there really isn’t anything topical that will actually help the stretch marks.
If your stretch marks end up lighter or darker than the surrounding skin after a year, you can explore things like laser treatment or skin tattooing. In the meantime you can minimize the chances of significant discoloration by protecting your skin from the sun with sunscreen (even when you are wearing a bathing suit… fabric doesn’t block all the suns rays).