That is very strange. I strongly recommend you ask your MP to send an email for you. If they are doing a medical extension, it is because the medical report is the only outstanding reason for issuing your Visa. Since the medical files just go to RMO Manila, there is no reason it takes longer to check an existing medical file than to examine a new one. My medicals expired August 10. But in between there was a PPR, photos, IMM5669 (I believe this one is for additional information for the re-med), criminal record check, and (at a later date) a re-chest xray.
For me it has been 2 weeks since my wife's re-chest x-ray, and I hear it takes about a week for it to be sent off. Not sure if it'll take an additional 4-6 weeks to re-examine the medicals... Hopefully sooner than that. But yeah, you should ask your MP to find out whats going on. Maybe your file is stuck.