HSG - trying after?

I had a HSG today which I found incredibly painful. Didn't get told any results so completely in the dark. In the leaflet from the hospital it says:
Side effects: 

You can resume your usual activities the following day. We strongly advise you to use contraceptives until after your next period to avoid pregnancy. 

Does anyone know why this is? I can't find anywhere on the net that says you can't try for that cycle.