What Causes Spotting After Intercourse?

hiya i have a two year old child after i had him i went onto the depo injection i come off in febuary this year and my periods returned to normal straight away having 7day period then last month i was a week late on my period my gp told me this is normal and that i could not get pregnant for 12-18months and not to be suprised if i dont have another period for 6months but when i come on it lasted two weeks however this month i come on as normal then i only had a two day period which wasnt as heavy as my normal period and after them two days there was nothing except slight spotting after intercourse what could this be?