Girl, I've got them too and I'm not fat either. My stepsister is a bean pole and even she has them. I think it is just a part of female life. Plus, you are tan, so they will be less visible. I'm white as a ghost >.> There are creams and you can buy to help reduce their appearances over time. Some of the best ones are probably for pregnant women. It is just a part of life, and try not to be self conscious of them. If anything it should tell guys that you've got a chest and a booty ;] They will fade over time too, just like any scar. In the mean time, try to love the skin you are in :]
There really is no cure unfortunately. Some people say cocoa butter but I've tried it and it doesn't work. They will get lighter over time but in certain lights, you will still see them. The person that finds a cure for them will become a multi-billionaire because everyone hates them. At least you have some dignity and won't walk around with them showing. Now that it's summer, I see some people baring much more than should be legally allowed and I cringe.