Sorry to hear that you've been experiencing itching! Most of the time it stops by itself but sometimes the culprit is infection.
Itching before periods is quite normal because the secretion of vaginal discharge decreases. Itching after sex is normal too because sperm is alkaline whereas vagina is quite acid which can cause discomfort or itching.
Make sure you use breathable underwear (materials such as cotton) and avoid harsh soaps in vaginal area. Wash yourself once a day with plain water but don't spray water straight to your vagina. This washes away the good bacteria and might make the itching worse.
If the itching continues or worsens it might be good to visit a doctor in case of infection.