Type 2 does anyone else have this experience?

Hi there - I was diagnosed about 6 years ago and started on Metformin. About a year ago my GP switched my Metformin to slow release, which I take with my evening meal. What I have found happening on most days, is that the following day when I go to the loo, the tablet has passed through my body intact!! This is quite concerning and I am thinking of requesting a change back to normal Metformin when I next see my GP. I have mentioned to him that this has been happening once before, but he just looked at me as if I were mad and questioned what I had said as if I must have been mistaken, which I definitely am not. I'm just wondering if anyone else has had this experience.