Ewing sarcoma help!!

Hello my friends!i notice my left leg (femur area) started swelling in the ending of November 2011, I was a high school senior at the time, so I was doing a lot of weight training, so i didn't think to much of it, then the pain started, so i went to the hospital, they did scans and a biopsy and diagnosed me with Ewing sarcoma February of 2012 started chemo right away, did 17 cycles of intense chemo and surgery in between chemos, I finish chemo on November 23 of 2012, then when I think I'm ready to resume my regular life I noticed a small lump on my left leg on January 2 2013. Met the doctors and they want me to start all over again. My case is very weird because all the scans I did on the 21 of December 2012 showed up negative, I did a pet/ct scan of body , bone scan of body, MRI of left leg, X-ray, chest ct scan, how come these scans didn't pick up the cancerous cells? I would think if cancerous cells was still in my left leg at this point, the scans should at least pick it up? I just don't get it? Is it possible for the cancer to to recur so quick? Within 5 weeks and 3 days of last chemo? And the weird thing is I didn't even see this new lump on my leg grow it just appear over night at one size. And this isn't the first time the scans didn't pick the cancer up, it happen when my left leg started swelling back in 2011. Another thing is that the cancer came back on the negative margine which was below the inital tumor site. this is weird i dont get it, im not even a bad eater, i dont eat junk food, rarely eat fast food, exercise, it's not like if I'm putting crap into my body to feed the cancer. Please I need some insight, did these events ever  happen to anyone else? I would  appreciate if you guys can answer my questions, and get back to me thank you all, much love.