Weight gain despite exercise

At the beginning of the summer I got a job at a daycare center which initially changed some of my activity patterns as I was made to stay up on my feet and chase kids around all day. Then I started jogging the trail (5k) behind my work during my breaks. Though I have made a conscious effort for almost two months to do more activity and exercise  during the day, I have been steadily gaining weight. Im not a little person, about 170 usually, but I've suddenly found myself at almost 185lbs! What's up with this?? Is it due to muscle gain from exercise or should I be more concerned? Help pls.