Help loosing weight - what am I doing wrong?

I've been slowly putting weight on the last while and nothing I do seems to shift it. I'm 21 years of age and I weight 9.11 stone and I would like to get back to 8 and a half or 9. I eat healthy and I always have done. I've always watched what I'v eaten since I was quite young. I've recently rejoined a gym but in the last week rather than loosing weight I've put it on. I suffer from a mild form of scoliosis that effects my lower and upper back so when I go to the gym I'm very concious that I don't want to do anything to put my back out. I've weakened it in the past from excersise and once it goes out I end up being able to do nothing then for weeks. I use to row alot in my teens but when I went back to it there a year ago I put my back out straight away and ended up in agony. I go to the gym about 4 times a week doing 15 mins of crossrunner, 15 of bike, 15 of tredmil. This varys from time. I'm hoping in the next few weeks to introduce other equipment and change my routine around as I build up my back. As my stomach area is the area I'd like to loose weight around and tone up as well as strengthen as I'm told it will help my back.. I'm told the rowing machine is good but I'm afraid to use it at the moment. When ever I do sits ups I always always strain my neck and end up in alot of pain. Any advice on what I could do to loose weight would be greatly appreciated. Might also be helpful for me to add that I normally head to the gym after work. This can be anytime between 5-7. I end up starving after work though and my stomach is rumbling at the gym. This happens even if I have a big lunch eg Baked potato and fish and salad or pasta and fish. I eat loads of fish. So by the time I'm finished in the gym I've a headache from hunger on top of it all. I'm at a total loss. I can't go home to eat before heading to the gym as the commute back would be too much as the gym is on my route home from work. ANy help greatly appreciated..sorry if I've rambled a bit just trying to get everything down

[painforsure]

I totally understand not being able to exercise due to back problems.  It gets so discouraging.  try to watch calorie intake and get a low impact yoga dvd.  This you can adapt to your abilities.  You may want to have your thyroid levels checked also. Hope this is helpful to you:)

[skittles81]

I also have a bad back. What I have been doing in the past week and a half  is the Zumba workout. The pace is pretty fast, but I do it at might own pace. When I started, I weight 144lbs with a goal to lose 28lbs. I have lost 5lbs so far. I am not going to lie, my diet is pretty bad. I basically eat anything, but  smaller portion. Some gym offer the class. I purchased the discs set ($70)

Ps. I do not eat out. So, I carry a bottle of Ensure in my purse. It keeps me going until I could get home.