I Have Been Taking Metformin Er 500mg 2 Tablets Twice

I have been taking metformin er 500mg 2 tablets twice daily. It is a solid pill. Been on this medicine for about a year. Today I noticed that the two tablets I had taken from the night before were in my stool. My blood sugar levels were normal. Why would this start happening now after a year of being on the med? The other medicines I am taking are being digested, but not the metformin. Any help would be appreciated