Not surprising, given that you're doing a lot of endurance running.

As Akima said, what you're doing is as important as what you're eating in whether it's muscle or fat that you're losing when losing weight. You'll inevitably lose some muscle, but the goal is to lose as little as possible.

In order to maintain muscle, you need to eat between .5 and 1g per pound of lean body mass, depending on activity. Endurance cardio is catabolic, meaning it burns protein once it's run out of carbs (because protein takes less energy to convert to usable fuel than fat). This is why the Olympic marathon runners are very often twigs. So you'll want to be on the higher end of that range.

If you really want to prevent muscle mass loss, then you'll want to incorporate muscle-building exercises (well, they'd build muscle if you were eating at a surplus, at a deficit, it deters the body from using those muscles for fuel), such as sprinting, weight lifting, or bodyweight strength training. The latter two are especially important for women to do, anyway, because they increase bone density and helps ward off osteoporosis (technically, running does, too, but only for the lower body).

Mark Sisson has a good article on who should have how much protein. His numbers are based on total bodyweight, though, so keep that in mind when reading it (I prefer the LBM-based method, because it's your muscles that need the protein).

http://www.marksdailyapple.com/how-much-protein-should-you-be-eating

I just had my last assessment for the year as well, and my body fat went from 21% in September to 18.9% just two days ago. I credit my lifting routine to this as it will burn off the stubborn fat and build a bit of muscle. For pound comparison, in the 3 months since my last assessment I've only lost 1 pound but as you can see from my body fat percentages, I'm certainly on the right track.

There's no reason that as a runner you can't also do some weights to try and turn the fat that is left into some useful muscle. Leaning out can only be of benefit to you. Strong lifts program is where I started and in the new year I plan to move onto another program I found online. And I'm not lifting anything impressive, but I'm lifting enough to keep my body lean and keep the body fat percentage down.

Hope this helps.

Actually NOT doing distance running anymore! Have been focusing on weight training and short cardio. Haven't run distance since June....

Fair enough. Whether it was a factor, then, depends on when you last had your body fat measured. If the last measurement was this time last year, for example, then it may have still influenced the change. If you've measured it since June, of course, then it's not.

It's hard telling, then, what may be the issue without knowing what you're eating and what workouts you're doing.

Your protein should be at about 1g per pound of lean body mass, and your strength program should be a consistent one with a progression mechanism (ie - doing 50 situps and 50 pushups every day, without increasing the number or difficulty, isn't going to build muscle in the long run).

It might also be worth considering decreasing your deficit, or eliminating it altogether. Judging by your pictures, you appear to be pretty close to a healthy weight. If you eat at maintenance and work on your strength training, you should see your body fat percentage go down in future check-ins, even if the number on the scale doesn't change. Strong Lifts and Starting Strength both have good programs if you don't have one already.

I just did a quick google search and saw that measurement error with the skin fold test is plus/minus 2 to 5%. Was this the method they tested your body comp? If so, this may be your culprit, possibly?