is there a way to...

to get a lost period back without gaining weight?

that depends on why you lost if.  If it was from undereating and subsequent malnutrition, then no.

If you've lost your period because you're underweight it's generally a sign you should gain weight for health reasons. I guess if you increased the percentage of fat in your daily diet it might help- more olive oil, avocadoes, nuts..

oops, I just posted a replay on sm other thread. There's a medicine (hormones) out there called chlormadinone. It  wont make you gain, but i'm no t pretty sure if you need a doctor prescription or if you can just buy it at the pharmacy. x)