I've taken it before (more than 10 years ago) and I'm on it now.
It didn't affect my period then or now.
I was pre peri-menopausal back then and everything was as regular, as long and as heavy as usual.
I'm peri-menopausal now and I can go about 70 days between cycles (this was before I started up with phentermine again last Dec) and nothing has changed period wise.
Weight wise I'm down 40+ lbs and as per my Dr I'm sticking with eating 1200 calories per day and keeping up with my water intake (at least eight 8 oz glasses of water per day).
See your ob/gyn.
It might be just your PCOS acting up or not.
My friend had a period where the bleeding got longer, heavier and gradually got more frequent to the point where it became continuous and she became anemic.
It turned out to be an early symptom of breast cancer.
She had a lumpectomy, did chemo and radiation treatments and took medication to put her into chemical menopause for 5 years (her type of cancer fed off hormones so they suppressed them).
She's fine now and has been cancer free for over 20 years.
Phentermine never affected my cycles but people are different and I can't say that it's not affecting yours.
I had horrible side effects when I took cholesterol lowering medication but other people take it and never experienced what I went through.
Make an appointment with your ob/gyn and get it checked out.