Congrats on your 15lbs!! That's fantastic!!
I noticed that, as I'm losing weight, I'm seeing more of my stretch marks, and possible getting new ones. I say possibly because I'm not sure if they are actually new, or if I had just never seen them before (my chubby hid them!). The good news is they do fade! There are some on my stomach that are almost invisible now, and I don't do any type of tanning.
Make sure to stay hydrated, and moisturize your skin. I use Jergen's firming lotion (I like to believe it's going to help my skin stay tighter!), but there are tons of lotions and creams that have strong moisturizers and vitamins to help keep your skin elastic. Age also plays a big part, too! The lotion won't help the stretch marks disappear, but it should help prevent any new ones from forming.
Hope this helps!