So my friend has a weird monthly phenomenon....

So im not really sure how to word this but ill try my best to describe it, one of my best friends came to me today and told me quote, "dude i dont know whats wrong with me but like every couple months i just get really gay for like a week or so and be fantasizing about gay stuff and idk why it happens". I wasn't really sure what to say to that so i asked for more details and he said in addition to the previous thing," yeah it kinda be like when im with a girl i wanna be a dude but when im with a dude i wanna be a girl." I honestly dont know what either of these things mean but i hope someone on here can give me a little help with how to respond/help him.