Started running C25K and continue to gain weight

I am beyond frustrated. I was able to keep off 60+ lbs for 9 years and now in the past 12 months I have gained 25lbs. I have health issues, so I went for being sedentary to jogging 2-3 days per week doing C25K. I am in about week 6 and I am still gaining. No, it's not muscle, as my pants are getting tighter. I eat roughly 1500 calories per day. I am limited on the amount of fruits and veggies I can eat as they mess with my GI system. I am post-menopausal (at 43) and am on hormone replacement therapy since a year ago. Any thoughts or ideas as to why I am not losing? Thanks.