Thank you for your advice, the problem went away without any medical attention but I'll be sure to visit my GP straight away should the problem ever arise again :)
I agree with both of the above two replies.
From your description this is probably a very small anal fissure that will heal by itself within a month. It will get worse (larger) and more troublesome if you do not allow it to heal properly. If you bleed any more you MUST see a doctor.
Typically an anal fissure will produce very fresh red blood whilst defecation or anal sex is taking place. This blood would look just the same as if you had cut your finger. If you are noticing (in the bathroom/toilet) that you have passed darker blood then this could indicate other conditions and you would be wise to see a doctor as soon as possible.
However, it sounds as if you quite enjoy anal intercourse and that you will continue with it after you have given yourself time to heal. If that is the case you need to concentrate on fully relaxing your anal muscles both just before and during anal sex.
The anal sphincter (the ring of muscle that keeps the anus closed) is a very powerful muscle indeed. If you can relax it fully (when a penis is in your bottom), without squeezing back up again, you will cause less friction and pressure on the internal walls of the anus and therefore limit the chance of fissures reoccurring.
Fully relaxing can be pretty difficult - but not impossible:
Create the time and the place for you to get into the right mood.
Have lots of foreplay.
Use lots and lots of lube.
Insert a finger (yours or his) into your anus very slowly and gently and allow your anus to relax.
Be very sure that fingernails are really short and have no sharp edges.
Use lots more lube and push your muscles out a little as he pushes his penis in and this will enable your sphincter to open up widely enough for deeper penetration to take place.
If anything hurts then stop.
Insist that he goes slowly and NEVER does anything suddenly or too forcefully and stops immediately if you ask him to.
Trusting him will help you with your relaxation.
He should only speed up towards his climax when you are fully used to all of the sensations and, hopefully, enjoying it too!
If you do not squeeze, anal sex will still be nice and tight for him and it should be more comfortable for you too!
I know that it is not for everybody but I am a bisexual male with 40 years experience of all forms of anal sex with both men and women and I base my advice upon that experience.
Good luck!
I'd avoid anal for a month or more to give the area an opportunity to heal. If the problem is still apparent after that, then yes, it may be worth seeing a doctor.
You may have an anal fissure. You should probably see a doctor about it.