First, let me just say that it sounds like you're doing GREAT with eating well and exercising, so congrats!!
If you're just starting out, sometimes weight loss is really unpredictable the first few weeks.. some people lose a ton immediately, some don't lose at all for a month. You just need to trust that you're doing the right thing and stick with it.
The constipation is probably part of the issue... you could easily be holding a pound of waste that you didn't have last time you were weighed.
Is it possible you were a little dehydrated the first day?
Could it be hormonal? A lot of women gain weight before or during their TOM.
Do you track your sodium? Were the past 2 days any higher than usual?
Have you slept less? More? Caught a cold? Been especially stressed? Spent more time in the heat than usual? There are sooo many things that can impact your weight (usually just due to water retention). Two weeks isn't really enough to know whether your plan is working for you or not.