Cannot understand my weight gain

Hey everyone,

I am new to this forum, so hello to you all.....I am hoping that you guys can give me some much needed advice.....
I am 30 years old, and had been very slim all my life - that is until now
I am roughly 5ft 5, though people always think I am taller than what I am for some reason??
My weight before now has been anything between 8 and a half stone to 10 stone, and that range I'd imagine is about right for me.
For the last 5 years I got into a kinda rut of avoiding the scales, partly coz I knew deep down I was putting on weight and didnt want to face up to it, and knew how I would feel when the scales tipped right over lol
However, after being on holiday and seeing photos, I decided to bite the bullet and weigh myself. I weigh between 11 and a half and 12 stone.
Now, whilst I knew by the fit of my clothes that I was defo putting on a bit of weight, I honestly didn't think I'd put on too much. I am a size 12, and don't really look hugely overweight. I have checked weight charts and I am defo overweight even for the larger framed person.
My problem is that I have been a vegetarian most of my life, and try to eat healthy. The noly thing I can think that has caused this weight gain is getting older and not enough excercise. I have never really been overweight before, and am at a bit of a loss at where to start with losing it!!! Esp since I am a vegetarian!!!

Any advice would be greatly appreciated.

Many thanks,

Carrie x