Okay, so im usually a pretty active girl since I started to lose the weight ( 15 kilos / 40ish pounds over 6 months). as in, gym 4-6 days a week, cardio and strength. plus, mountain biking / bmxing and a bunch of other stuff.

About 5 weeks ago I fell ill with bronchitis and had to completely stop all activity. I only gained 2 kilos (5 pounds ish).
Now, im in my second week of training again and im finding that I have an insatiable appetite. yesterday i packed in an extra 1000+ calories and could still have eaten more. today, i feel the same and feel out of control.

I usually aim for 1500 calories since i want to lose about 20 pounds, but I think that this may be a little low - my active metabolic rate can sometimes reach about 2500.

Anybody have any suggestions about what could be going on?
Is this a reaction my body is having to low caloric intake?
Thanks =)

(If it helps, I'm 5'4" and 149 lbs / 68 kilos)

Do you feel like your energy levels are constant or are you getting peaks and crashes?

1,500/day isn't very much at all for a very active adult who - I can't remember the healthy ranges right now, but I think this is right - isn't very overweight. The body is bound to react at some point. best I can do is reassure you it isn't surprising this happened, there will always be an opening of floodgates, so to speak, after a period of restriction, at some point. Your best bet might be to drop the counting and focus on reasonable portion sizes, eating when hungry, stopping when full, and healthy food if you want an added focus.

I have found in the past that really upping exercise makes me famished.
I am not sure how fast you "need" to lose your weight, but upping your calories to 2000 for a while may really help. You can always get them down again if need be. Or even do a week at about maintenance (closer to the 2500 you are burning in a day) and then go down from there.
You must be awesomely fit. good for you