You probably wouldn't develop anal fissures from a vaginal birth- that is more related to something coming out (or in as the case may be:)) of the rectum. Most likely it is a hemorhoid, which is normal and not unexpected in pregnancy and birth. You can see your doctor, or the best at home treatments you can do are witch hazel on a cotton ball to the hemorrhoid, soaking in a warm bath, and if you are not squeemish you can gently replace the hemorrhoid in the rectum while you are in the shower or bath which helps with the symptoms as well as healing. There are not a ton of other things a doctor can do except surgery in a really severe case.
To help heal, avoid constipation by getting plenty of fiber and water in your diet. Get plenty of exercise to keep your system going, and kegels can help by strengthening that whole area.
Good luck- this is one of this little talked about horrors of motherhood but as an OB nurse (and mother of two) I can tell you it is extremely common.