Heavy periods. Greetings. first of all!

Hello respected Doctors Greetings. First of all thank you all for your seevices. I am 39 yrs old unmarried female. For the past two yrs I have been noticing that during winters only my period gets heavy and too much cramps. I have been doing my usg and complete MRI to rule out anything like cysts or fibroids. But the report comes clear. My thyroid values are always normal. I am not diabetic either. From the month of Dec till March my period gets all messed up. After which it becomes normal. I am not understanding what's going on. I have been to my Gynae but she says everything is normal. Your input would be highly appreciated. If all my values are normal and I don't have any cysts or fibroids then why this is happening? Regards Ishani