As a female (I'm guessing one of the few actually responding) let me tell you that your weight is not the issue. It is as many people have already said, a calories issue. If you just increased your mileage you are likely not eating enough. Talk to a nutritionist, count your calories... you may be surprised at how little you are eating. Have a dessert, talk to a nutritionist.
If you find you are eating enough, go to your OBGYN (they will likely tell you that you aren't eating enough the minute you walk in) and get a prescription for birth control pills. These will help regulate your hormone levels and ensure that you have hormones levels that will not lead to osteoporosis. (it also can be handy to predict when your period is coming or time around big races.)