Good week - maybe not

Hubby and I joined our 12 week community weight loss challenge at the health department. I had my first weigh in and was surprised. Not pleasantly surprised!
I weighed myself at home on Wed. with a loss of 1.5 lbs. I thought when I weigh today I would have a loss. Nope! I gained 1/2 pound. I had a very good week eating healthy and exercising.
A lot of explanations went through my head: wonder what I ate that had so much sodium last night? or Must be water retention.
My body is reacting differently to weight loss than before. I have accepted that fact - I guess. I won't give up and tomorrow starts another week for the 12 week program.
(Hubby weighs in tomorrow. We will see how he does!)