I don't know if you are losing TOO fast or not. For someone your (and my) weight I don't think that anything up to the range of three to four pounds a week is unacceptable - if it's being done right. Five pounds is probably pushing it a bit but probably not by a huge amount. The key is in how you are achieving these results - IF you are truly eating enough calories (probably something on the order of 2500 at the mininum for you) and you are exercising vigorously, then 5#/wk for at least a good little while is probably not too dangerous. What you have to watch out for are three things - if you are losing this fast because you are not eating enough calories then you might be burning up lean muscle. That's not good. Then, even if that five pounds is fat (and is likely more if you are eating enough and building muscle from all the exercising) you have two concerns. One is from the various toxins and hormones that can get stored in fat cells and that get released into your system when the fat gets burned. If that happens at too high a rate it can cause problems. Being sure to stay very hydrated will help out there. The other problem is the adjustments that your body has to make in response to weight changes and the limits on how quickly it can make them - particularly things like your electrolyte balance.

As long as you are feeling good and are SURE that you are eating enough to meet your BMR calorie needs and are staying hydrated you will probably be okay but I would keep an eye on things and be prepared to slow your pace down if things seem to be going astray - and I would also expect your weight lose to slow down anyway of its own accord.

Now, having given you all these pearls of wisdom, you have to take them at face value knowing that I am NOT a doctor or in any other way qualified to give firm, expert advice in these matters. This is based on my experience, my readings and my interactions with others. As always, the best advice comes from a physician (one that is LC-friendly in general) and, if you want to maintain that type of weight loss for much longer, I would recommend seeing one. If nothing else, it will ease your own mind to get your electrolytes checked periodically.

Best of luck to you.