All of a sudden, cant get 'it' up.

Hi,
Im having a problem the last 3 days thats starting to worry me.
To start off id just like to say that Im only 18, and ive been regularly sexually active since i was 15, and nothing like this has happened before.
in the start like 10 days ago I was having sex all of the time with my girlfriend. before sex, just making out for a couple minutes would have it up & ready to go. that kept happening, then about 4-5 days ago i started cuming kind of early for some reason, about 10-15minutes in. I felt that it wasnt as pleasing to her as much as before, so about 3 days ago I tried masturbating before she came over thinking it would make me last longer. Then later when we were about to have sex, it wouldnt get hard. she started playing with it, and even that wouldnt work. note that before it would just from kissing before sex, it would be fine. Now it cannot get it up all the way at all. It will start to and get semi - hard but not all the way up. even porn will not work. i tried while it was as up as i could get it and it was working a little bit but wierd, and still wouldnt get up all of the way and it slowly went back down. Im only 18 and this was just one random day. idk what its from and i reeeallly need help.