A few boob/bra questions? Lol?

im 15. I have very small boobs. I don't even have boobs. When I don't wear a bra you would think I was a guy, so I really don't have to support anything. I seriously think I'm less than an A cup. Anyway, I bought a semi cheap extreme push up 36A bra from rue 21. Cheaper than Walmart but isn't a Victoria secret. It was comfy, and made my boobs look decent. But since I have nothing to even barely fill them I put a sports bra over it to hold it down, cause the tops kinda stick up. Is this okay? I heard wearing a bra that isn't perfect for you can give you breast cancer. I just bought a new bra today, same style. Extreme lush up and an A cup but it was a 34. It's pretty tight even on the 3 noch. I'm gonna buy 3 more of the 1st kind I had. Is this safe? I doubt I'll wear the one I got today bc I told want breast cancer and its too right. Is it ok if I stretch it out and it's comfy if I wear it? Is it ok what I'm doing? I hate to waste the $18 cause I'm cheap, but I don't want cancer.