Hi and welcome to the community. Weight loss can be a sign of cancer, but can be caused by other things. In my case, I lost a lot of weight prior to being diagnosed and it turned out I also had pernicious anaemia which was easily remedied. It may be worth while talking to your GP about it. Best wishes.
Hi hope you are OK 
I was diagnosed with cis in February just started bcg treatment had 2 so far .. I lost a stone in a month or two but I know my appetite had gone due to me stressing and feeling anxious about having bladder cancer...I had it to lose but you do start to worry over the loss ..I think we spend so much time worrying over everything when we have the diagnosis..I still get my times when I feel down about it all but thankfully nothing like I did at first ..I've read so many positive things on the forum and they've helped to keep me positive 
lots of good advice and encouragement x take care and best of luck 
