Problems getting hard around my gf, please help?!

Me and my girlfriend (who is a virgin) have been dating for a while now. She is 18 and I'm 20. We have been fooling around a lot lately. Like 2 weeks ago my gf started to give me a hand job but my penis wouldn't get hard! She tried so hard and nothing happened, I had been drinking that night though so I blamed it on that. Last night, We were fooling around again and she got me all hard and turned on. I fingered her and we stopped for a while. An hour later she was still horny and she told me she was ready for sex. So she tried to get me hard, and she did for a bit but then I put on the condom and I went soft! In the morning, we fooled around again and she got me really hard but we couldn't do anything cause her sister was awake!

Now, I have NO problems getting erections when I'm masturbating to porn. Also I tend to get a lot of random erections. Sometimes she can get me hard, but there are just times where I can't get hard when shes trying and I have no idea why! Can anyone explain this? thanks :)