Just recently joined and was hoping to get your opinion on this past week. In the past week I went from 162 to 166lb. I track all my calories and have been eating 1400-1900 calories each day. I have done two things differently this week. I have done strength training for an hour two days. I haven't really done strength training prior to this. On the other days I have ran five miles each day. The second thing I have done differently is started taking iron supplements.
Is it possible that either of these two things could result in weight gain? I am 100% sure my calories are accurate so it just isnt making sense that I would be 4lb heavier in a week.
In addition to gaining a pound of fat by over-eating 3500 calories, there are many alternative explanations (in no particular order)
eating heavier foods (like vegetables)
accumulation of matter in the colon prior to a bowel movement
glycogen level (1g of gylcogen is stored with 3g of water)
water retention due to eating more sodium than is typical for you
Personally I am not a fan of unexplained fluctuations so I am a member of a free website http://www.physicsdiet.com where I can plot my weight. There is a built-in feature of an "averaged weight trend" which is very smart at allowing me to ignore fluctuations and see the overall trend
You mentioned you have started taking iron supplements, these can cause constipation so that could be it. The strength training has probably increased glycogen in your muscle. Increased glycogen means increased water retained. In addition to that muscle weighs more than fat..