Thanks for posting,
Your doctor is probably right about the folic acid
supplements as they are no known documented facts that associate or significantly correlates folic acid intake to oligomenorrhea
(period of >35 days). You might have developed a new pelvic diseases or hormonal imbalance or psychological (stress, emotional instability) or lifestyle change (diet, exercise, travel etc) or even tumors (prolactinomas
of the anterior pituitary
gland, PCOS) or even drug that might responsible for the prolonged periods
(oligomenorrhea per your description). After listing the possible causes of your problem, you can decide on your own which one might be implicated or you pay a visit to a gynobs for can do a proper review and provide adequate treatment.
Hope this helps you