Metformin and late period

I have PCOS and high insulin levels. My periods used to be extremely irregular, heavy, long lasting. I have been on metformin since almost exactly one year ago. The last time I saw my dr which was in January, we upped the dosage. My cycles are 35 days long since finally being regulated with this medicine so we went up in dosage twice now to see if they would get any more regular than that but no change until this month. I’m not sure what’s going on. My period was supposed to start march 31st (36th cycle day) and it’s now 4 days late. I tested the 31st, April 1st and 2nd and got bfn on all tests. Still no period and really no symptoms at all besides some nausea which could just be from the medicine and boobs are only slightly sore when touched. I have had my period on the 36th cycle day every single month for several months now but I keep getting negatives and my period isn’t here?? At what point do I rule out the possibility of pregnancy? When should I call my doctor?