There can be quite a few things that cause rectal bleeding - and most of them are relatively benign. Things like piles, or even over zealous arse-wiping, can cause blood spotting.
If it is relatively fresh (bright red) blood, and in small quantities, then that tends to be a good sign.
But you're doing the right thing discussing it with your GP. However, don't be surprised if he wants to pop a finger up, or if he gives you a pot for a sample.