I'm thinking water retention as well. You are probably drinking more water than before, and if you had anything salty before the weigh in your body is going to hold onto water, no matter how carefully you track. And I regularly gain lbs around my period, but I've learned not to stress about them because that weight will go away on its own.
Oh and I just remembered, I gained weight the first week, the very first time I joined. I had surgery between the first and second weigh-ins and the meds just made me retain water. It happens.