Weightloss has caused some medical problems, but i still wnat to lose around 5-10 lbs, what can i do?

okay so last year in May weight 152 lbs i now weigh 120, i am 5'2, female, 16. I haven't had my period in over two months yet, i dont know why but its sressing me out because i am still determined to lose 5-10 lbs as 115 was my original goal. I have a small frame so it is ideal and appropriate for me. I do step aerobics daily but I stopped for the last two weeks due to the fear of my health, anyway....i usually did 60-80 min of step aerbics, and when i say step all i mean is STEPPING UP AND DOWN, i can't do that stuff because i have weak feet since i was a toddler. All i want to know is how long can i do that excercise for a day and still lose wieght oh and i'm on a 1500 cal diet. Btw, I cannot go jogging because my neighbourhood just isn't made for it, if you know what i mean..