There are a huge number of effects that are caused by being underweight, but first the question would be why are you underweight? Do you eat a balanced, sufficient diet for the level of activity that you do? Do you ever stop yourself eating orthrow the food back up again in order to remain underweight? Is your BMI (not the best indicator but it is simple) or a healthy value? I think what I'm trying to say is do you have an eating disorder or any of the symptoms of one?
If you don't and are just naturally slim, eat a good diet then your period loss is probably caused by the stress of going to university however I don't think that your doctor should have just put you onto the pil without first trying to determine the reason behind why you have not got periods.
As fot the effects of being underweight: immune defences decrease, changes in blood chemistry cause mood changes, bad memory and fainting, hair thins and eventually falls out, low blood pressure and slow heart beat, eventually heart palpatations and failure, anaemia, weak muscles, damage to the bones makes fractures and osteoporosis more likely, kidney failure and eventual loss of bladder control, constipation and eventually loss of rectal control, periods stop, difficulty getting pregnant in the future, if still growing then stunted growth, due to the fact that you cant regulate body temperature you grown fine downy hair all over the body, gum damage due to a lack of nutrients, if you throw up then the stomach acid rots your teeth which leads to fillings and extractions.
I know i paint a bleak picture but these are the effects. If you do have an eating disorder and need someone to talk to who has been there and come out the other side then feel free to pm me and i will give you may msn.
sorry for long post
x