Drug test tomorrow HELP

Okay so this is my first post. I'm an occasional smoker and for the past year (ever since I started smoking) there's always been a 1-3 month gap between every time I've smoked. It just happens to be that I've smoked 4 times in the last month. One hit on 5/18, couple hits 5/25, a couple 6/10 and the last time I smoked was 6/16. I have a drug test tomorrow 6/26. That gave me 10 days for my system to clear. I found out about the test last week, it's for the nursing program at my college. I get kicked out if I fail. I wasn't aware at all that a drug test was a requirement. Starting last Thursday the 20th, I started taking 500mg of flush-free niacin a day. I also drank a gallon of water and 20fl oz of cranberry juice. I also went to the gym 3 days in a row and did some pretty intense cardio. I also went to the beach and tanned for like 4 hours, making me sweat like crazy and biked up and down the boardwalk a few times. I bought a drug test kit at walmart and took it saturday the 22nd, which was 5 days after i last smoked. in about 2 minutes it showed up negative with a bold line, and i made sure my urine wasn't diluted. I eat pretty healthy, except for a few snacks here and there. I'm 18 and 5'2" and weigh 112lbs. I'm a dancer so I have a very athletic build and a very low body fat content. I also take a multivitamin, a b vitamin, c vitamin and calcium supplement every morning. Any advice/input would be very helpful. I don't plan on using any other urine than my own and I can't push back the test. Oh and also, if i fail, how long does it take for the lab to call and tell me that? i heard that they dont call if you passed. Thanks!