Weight gain after valleyfever?

Possible. This is not a feature of the disease itself, but may accompany any protracted illness. You may have lost some weight with the illness for a variety of reasons and are now regaining to your norm. If wt gain is excessive see you doc.
Yes. Valley fever is a stress on your immune system and the stress response, when chronic and in excess contributes to various hormonal changes that can lead to weight gain. Cortisol, insulin, estrogen and leptin can rise, while thyroid, testosterone, progesterone, adiponectin, melanocyte stimulating hormone drop. These changes are mediated via cytokines.