Why am i gaining weight ever since i started running?

Hey all! So long story short I have been riding (horses) my entire life and since mine died last year :( I haven't been able to keep up with it so I have had to start running to maintain my weight. At 5'2.5" I'm 107 pounds right now. I was at about 104 for several years with just controlling what I ate and riding every day (which was not an intense work out, to say the least). I'm only 23, so I can't blame metabolism probably, but ever since I started running (about 30 minutes a day, 6x a week) my weight has increased and my pants hardly fit in the butt and theighs anymore. The most common reply I will get is "you are subconsciencely eating more because you are working out more" but I have been counting calories since I was 17 and I'm sticking to about 1500 a day, I know how not to eat extra (even though it is SO hard). So, my eating hasn't changed, the only difference is I am running now instead of riding. Why do I feel so big and heavy and why do my pants hardly fit when I have only gained 3 pounds? And how do I lose the extra weight? Thanks so much everyone.