Change in cycles as we age


I just turned 39 last month. And my cycles used to be 30 to 33 days. Sometimes longer. Last few months they have gotten shorter. 25 to 29 days! And i haven’t gotten a positive OPK at all this month. I’m now CD 21. Could I be having the start of early menopause?? We really wanted one more baby (I have 3) but I think my body is done.