Hi thanks for your question.
The itching and redness
your having on shaft of your penis could be due to allergic reaction
to the chemical present in the douche,
since your girl friend also having complain of itching ,so chances of allergic reaction are more.
Yeast infection can also produce itching but yeast infection will also produce dischage.
I would suggest you to get examined youself and your girl friend by a physician to know exactly what is the cause of this redness and itching.
Hope this answers your question.