help needed about weight gain

Hi to you all.
I have been an avid follower of the gym for 8 years and taken part in classes ie, step n tone, circuits, body combat, aerobics etc. I have never really changed shape over the 8 years probably due to a poor diet and drink, but recently I started to go into the gym to do the weights as well as the classes in hope to shape and tone by body more.
To my horror I have gained weight? I was 62kg and am now 63.6kg...and thats only in 2 months??? arrggghhhh how come im getting bigger? I have notice that I now hang over my trousers and they are tighter and I have a jelly belly.
Anyone out there who can help tell me whats going on.....middle age spread (im 37) I AM NOT PREGNANT EITHER!
xx