Gaining weight while eating well and exercising...

I recently started back with an exercise regimen. My goal is to lean down and build muscle, particularly in my upper body. I have been exercising and eating well (around 1500 cal. per day) for a couple weeks. My exercise is generally brisk walks and/or circuit training videos. I eat frequently and focus on lean proteins. The problem is, no matter what I eat or how much I exercise, I am gaining weight. Lask week I gained 5lbs and my clothes feel tighter. Obviously I haven't gained 5lbs of muscle in a week, so what gives? I am very diligent about logging all of my food into cc, so I am confident in my caloric consumption. I suspect that I am probably off on my exercise estimations (as far as calories burned), but not to the tune of 5lbs! 

Rapid weight-gains are usually related to fluid retention rather than fat or muscle.  Can be associated with extra exercise - also salty foods, sugary foods, caffeine, alcohol, lack of water, lack of fibre, too many processed/packet foods.  Fluid retention is usually temporary.

If you're confident that you're not underestimating what you eat then have a look at the quality & balance of your diet.... Maybe cut back on the meat/eggs/fish/dairy (protein?) and eat more fibrous foods like vegetables and grains so that your digestion can remove the waste products from your body.  Drinking more fluids could help as well.

There could be a couple of things to consider. I don't know your stats so I'm not sure if 1500 calories is good for you but aside from that. How is your sodium levels. Too much sodium will make you retain water and could cause your weight to go up. Also, do you do cardio too or just weights to build muscle. You should be doing both.

I know that I have a thyroid condition and really struggle to lose. Have you ever had it checked. I wouldn't worry about it too much right now but it may be something to consider if this becomes a consistent trend for you.

Good luck

Lots of people who begin excercising think they can eat more

and overeat thus gaining weight..

i'd try to watch more closely how many calories your taking in and substitute some chicken for some low calories veggies (green beans, radishes, etc. have low calories for lots)