What Causes Abnormal Weight Gain Despite Managing Diet And Exercise?

Hi. I have been finding it very difficult to lose weight recently. As far back as I can remember, I used to range between 54 and 58kg which is adequate for my height of 5 2. However, since last year the range has gone up tremendously to 63-65kg. I am a student and have always been quite active and cautious about what I eat even though I slip up occasionally. Since the beginning of the year I decided to take it a step further this year by joining the gym at this point I was 64kg. I have been doing multiple classes a week and literally working my butt off doing both cardio and strength training. Today I got on the scale and found that I am now 68kg. I do not understand what is going on and whether this could be a more serious problem.