There's already a ton of great advice in this thread, but I just wanted to contribute an idea which is depressing, but true. Your metabolism WILL change at some time in your late-20s to early-40s. I have no idea how old you are and if I guessed at your age through pictures, I'd probably say somewhere in your mid-20s.
It's not the end of the world, but it does manifest in the way that you're describing. My big metabolism change happened at the same time I had a serious back surgery. I thought the inactivity associated with the back surgery had was responsible for my weight gain. But after I began working out again with my same regimen and eating *less* than in my pre-surgery days, I was still holding on to my weight. I had been extremely active my entire life so I was very familiar with the way that my body managed weight. Or so I thought. I had to learn a new system to control my new metabolism.
The way to combat it is to aggressively change your schedule/habits to jump-start your metabolism. So, this is where all of the good suggestions in the thread come in: aside from your normal work-out routines, get in 10-15 min of cardio first-thing in the morning; ingest most of your calories in the morning; eat small meals more frequently, and consider fasting (or almost no food) after 6pm.
Unfortunately, drastic metabolism change in women is not an urban legend. It seems to happen to different people at different times. My cousin (not blood-related) is 5' 9", 135lbs or so, has always been super-skinny and has never worked out to control her weight. She turned 39 and all of the sudden, she was complaining about significant weight gain. She didn't expect to have to deal with weight issues in her life and was shocked at the change.
makes me sad.