Erectile Problem

Hello, I'm a 19 year old male and the last week or so I've been having a problem acheiving an erection. But, when my girlfriend was home 2 weeks ago i had no problem at all achieving one. She won't be home for another 2 weeks so I won't be able to see if I can achieve one w ith her. I don't know if this has anything to do with the problem but I have a very strong pressure in my pelvis area that feels like it is associated with constipation. Could this have anything to do with my problem? I've also had a stomach problem the past few days that feel almost like a stomach flu. And my bowl movement today was somewhat of a white color.