You mentioned you'd just had your period. Perhaps you had water weight and when you got your period, you lost the water weight.
Anxiety has caused me weight loss many times....no matter how much I eat and I eat a lot. My friends are always astounded at how much I eat.
I am not familiar with the med but it seems that others know about it and say that could be the reason, since you gained weight on it.
I have a rule...no standing on the scale when I'm anxious. Not even a peek.