I've been given Glycerol Suppositories by my doctor as I'm constipated?

I've been given them today by my doctor as I've been constipated for a few days and it's hurt majorly - I put it in an then literally 2-3 minutes after I needed to go (baring in mind I've been needing to go for the past few days and nothing has come out and it feels as if it's just gathered inside) and I went and my stool came out in one huge bit. Did the medication work how it was suppose to? And why does my rectum hurt so much now?