Haha, you could be me!
I'm 5'6" and female. At the end of high school I weighed in at 128 pounds. By the end of my first year of college I weighed 105. I was never anorexic, I do not suffer from any body image issues, and I was not trying to lose weight. At 105 I did have problems with my immune system and I was lethargic and generally didn't feel that healthy.
5 years later I've never gotten back to the 130 range, but now hover around 110-115. At this weight I haven't had any issues with my immune system sucking, and I figure it's probably ok since I'm very small boned. I do still get a lot of "oh but you are so skinny" comments though.
I started cycling 2 months ago, and have been riding at least 5 miles a day since then, and usually averaging more like 15-20 a day. So far I've only lost a couple of pounds, and I feel fine. I think this is because my appetite has been increasing relative to my activity level (with some activities it tends not to, such as walking, and then I have issues with losing weight).
My advice would be just to watch yourself for a while. Make sure you get enough calcium, cycling does not work as a weight bearing activity and therefore does not help with bone density like walking, jogging and weight lifting do. Eat when you get hungry, and make sure you keep snacks around, and bring them on longer recreational rides. I find this is one of the hardest things for me to do with school, I try to keep granola and stuff around so that I can eat when I get hungry.
So far I have not had to resort to supplements or anything to maintain my weight. I just keep an eye on my diet to make sure I'm getting enough calories (you can use fitday for a week or so to get an idea of what you are eating and which foods are good choices).
Foods I like:
Yogurt (I try to get the kind made with whole milk, though it can be hard to find)
Chocolate milk
Granola
Chicken
Avocado
I hate all the energy bars (I've tried about 20 and they are almost universally too sweet for my tastes) and find that granola is a really good substitute when I'm on rides.
I've been considering seeing a nutritionist to try to get an idea of how healthy my diet is and if there are things I should change. It's frustrating sometimes that the only diet advice I can find is either for people trying to lose weight or men trying to put on mass amounts of muscle, neither of which are groups I fit into. Most people laugh and act like I don't have a problem when I ask for advice on maintaining my weight in a healthy manner. If you find a source of good advice on this, let me know!