Here's some info I found online while researching this very problem:
There are numerous mechanisms by which losing weight may trigger an outbreak of acne.

Dehydration – Many herbal preparations that assist the weight loss process contain diuretics that are meant to flush toxins from your system. However, this can lead to dehydration. The body responds to dry skin by signaling oil glands to secrete excess sebum, which can block hair follicles and create blemishes. Drinking an adequate amount of pure water will help your skin rehydrate. Also, taking Acnezine, a natural acne treatment, will unblock your pores by removing dead skin cells—a result of dry skin.
Stress – Acne is often the outward manifestation of a general stress condition on the body. Losing weight too quickly puts the body under physical stress, which may result in an acne outbreak. Slowing down the rate of weight loss to a reasonable pace will reassure your body that you’re not in danger of starvation.
Imbalance – When your intestinal flora is in balance, digestion proceeds smoothly. An episode of acne may be a signal that the bad bacteria outweigh the good. One solution is to add good bacteria to your diet by eating yogurt or taking a probiotic formula supplement.
Detox – The body stores toxins in fat. When you lose weight by exercising and improving your diet, those toxins are released into the bloodstream and cleared through the liver and kidneys. Losing weight quickly can overload the system, resulting in an outbreak of acne. The liver, in particular, can cause acne outbreaks when it is overworked or not functioning optimally. Drinking water will dilute the toxins and speed them through your system more quickly.
Hormones – Abdominal fat is not merely benign blubber. Researchers have determined that adipose tissue plays a part in the secretion of hormones and other endocrine substances, which cause inflammation and irritation. Getting plenty of rest is helpful as your body dissolves and disposes of your excess adipose tissue.

I found my face has cleared a lot since losing weight!! and my complexion is better.

One thing you might want to do is examine the food you're eating, sometimes certain foods are triggers for me and my skin, not enough water...etc

When I started losing weight in January I broke out like crazy with acne. Before that I had a clear face with no visible acne or scars. Even after losing a significant amount of weight and changing my lifestyle 6 months in, I still had acne problems. I'm not sure what it was, but I couldn't handle it anymore and decided to see my doctor. She prescribed my doxycycline for my acne and a topical cream. It has worked wonders, no more acne!

Huh, I never thought about it, but my skin did take a turn for the worse a few months into eating healthier. Great question!!!!

As soon as I started taking fish oil I noticed an improvement within a couple days.

I have acne again too.

I am trying to tell myself it's a detox. My skin is reacting to the toxins leaving my body. Hoping it will clear up soon.

I'm not sure of the reason why, but apparently changing your (cotton) pillow case nightly can help to reduce the appearance faster. I know a few people who tried it and it really helps. Just a thought!

i actually experienced the opposite problem, i started getting fewer pimples once i started losing weight. now i have about as much as i had before i started. i guess its different for everyone.

I am also someone that is impacted seasonal - especially with the cold weather. It is a side effect of the drying. I usually try and use a better face cream.

My skin actually became clearer but I gradually changed my eating patterns and up'ed my excerise...I'm sure once your body adjusts the acne will disappear :)