Why do I get stretch marks so easily?

I'm female and I've been getting stretch marks since I was 14 years old. I am soon to turn twenty. Over the years they've gotten worse and worse. At first they were hidable only in my upper inner thighs. Then they slowly crept down my legs over the years. At 14 I was 130 lbs, and now 150. It's not like I haven't gained weight...but its not been that much in that short period of time. Any small weight fluctuation causes these awful marks. Regardless of the fact I drink a lot of water, always putting those creams on my skin. Bio oil, tried rose hip oil, I have many palmers products I use on a regular bases, collagen creams. I've been doing this for a while all it does it make my skin softer and maybe the lines slightly dimmer but the true scar never "vanishes" Its still pink, or tan colored still noticeable. At the beginning of college I could wear skirts. I really like to wear skirts, but I can't now. A few months after college they appeared on my hips and down the backs of my legs as far down as behind my knee caps. I can't wear shorts, I can only wear really long skirts. Swimming is something I avoid. I've had people make fun of me. Honestly. I know something like collagen and elastin are supposed to help and I make sure those things are in the products I use, and I've seen it in supplements? Would supplements help my skin? I'm at a loss because I am not ugly! I'm even getting fit again and I never got really weight! I haven't weighed myself in awhile but I'm probably 145 now. I'm ready to go to a plastic surgeon but who is to say they won't come back? I know a lot of it's in genetics but is there any specialist I can go to or any reason that my skin is just so un elastic? What treatments really work? I heard that body skin peels, and that skin rolling thing can help scars and you can buy those yourself. But I would rather get the right kind of help than possibly making the skin worse.