3 month plateau

Hi, I'm hopping the response to this post will provide me with some perspective. I began dieting and exercising routinely at the end of January/ beginning of February this year, after a poor diet and next to no exercise. At 137lb, I joined a gym, tracked and recorded my calories everyday, and was netting 1200 daily, however was not eating back the calories I burnt. (Which would be around 400/500 calories through cardio training only) I was losing roughly around 2 pounds a week and was fairly proud/happy with the results. Round mid April, my results halted. I wasn't aware of what a plateau was at this point so I carried on with what I was doing, and still no change after 2 months which is highly discouraging.

I've introduced strength training these past few weeks, as well as attending circuit training and body attack classes at least once a week. ive also upped my calories to at least 1400 a day, whilst also adding more sources of protein though foods such as meats, eggs, cottage cheese, protein shakes etc. Through much research into stories of upping calories and breaking through plateaus as increased metabolism, for me it hasn't worked. I've not touched the scales though fear of disappointment, and can feel I've gained weight when I look in the mirror.

I'm a 17 year old female, and the last time I weighed myself I was around 120lb. I go to sixth form 5 days a week and go to the gym at least 3, if I don't go to the gym I go for runs or do strength training at home. Please help as breaking this plateau is all I'm able to focus on at the moment!