@August ****
, DON'T confuse the extension of stay stamp (which you got from the immigration office for 60 days), with how long the VISA is valid for. When you come back from Cambodia, your extension will be expired BUT the VISA will not be and you'll be stamped in for 90 days.
Just so you know;
You really didn't need to get ANY extension of stay 2 months ago, you could have exited/re-entered the country and gotten stamped in for 90 more days, because that's how that visa works.