Frequent breastfeeding when your baby is first born, usually means that you won't have any periods. However, when the interval between each feed incereases the likelihood of your periods returning is greater. Some people won't start menstruating until after ceasing breastfeeding completely, some it begins again much sooner. As with most things hormonal, there is no hard and fast rule!
I'm sure you can get information from your midwife, doctor, health visitor or other Mums if you're concerned.